Issue Description:

The compiler crashes attempting to compile this file:

test.swift

struct S<T> {}

extension S where T == Int {
    typealias Callback = (Bool) -> Void

    func process(callback: Callback) {   
    }
}

When I run `swift test.swift`, I get this crash output:

Assertion failed: (static_cast<bool>(genericSig) == numSubstitutions > 0), function get, file /Users/buildnode/jenkins/workspace/oss-swift-package-osx/swift/lib/AST/ASTContext.cpp, line 2960.
0 swift 0x00000001127f32e8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1 swift 0x00000001127f39f6 SignalHandler(int) + 694
2 libsystem_platform.dylib 0x00007fff59c00f5a _sigtramp + 26
3 libdyld.dylib 0x00007fff5997f249 dyldGlobalLockRelease() + 0
4 libsystem_c.dylib 0x00007fff59a2b312 abort + 127
5 libsystem_c.dylib 0x00007fff599f3368 basename_r + 0
6 swift 0x000000011014d823 swift::NameAliasType::get(swift::TypeAliasDecl*, swift::Type, swift::SubstitutionMap const&, swift::Type) + 563
7 swift 0x000000010ff64f27 swift::TypeChecker::substMemberTypeWithBase(swift::ModuleDecl*, swift::TypeDecl*, swift::Type) + 663
8 swift 0x000000010ff64af9 swift::TypeChecker::resolveTypeInContext(swift::TypeDecl*, swift::DeclContext*, swift::DeclContext*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, bool, swift::GenericTypeResolver*) + 553
9 swift 0x000000010ff6c8c6 resolveTypeDecl(swift::TypeChecker&, swift::TypeDecl*, swift::SourceLoc, swift::DeclContext*, swift::DeclContext*, swift::GenericIdentTypeRepr*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) + 214
10 swift 0x000000010ff6c482 resolveTopLevelIdentTypeComponent(swift::TypeChecker&, swift::DeclContext*, swift::ComponentIdentTypeRepr*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, bool, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) + 1698
11 swift 0x000000010ff66b34 resolveIdentTypeComponent(swift::TypeChecker&, swift::DeclContext*, llvm::ArrayRef<swift::ComponentIdentTypeRepr*>, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, bool, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) + 564
12 swift 0x000000010ff6652a swift::TypeChecker::resolveIdentifierType(swift::DeclContext*, swift::IdentTypeRepr*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, bool, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) + 138
13 swift 0x000000010ff671c3 (anonymous namespace)::TypeResolver::resolveType(swift::TypeRepr*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>) + 163
14 swift 0x000000010ff670ed swift::TypeChecker::resolveType(swift::TypeRepr*, swift::DeclContext*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) + 173
15 swift 0x000000010ff65acd swift::TypeChecker::validateType(swift::TypeLoc&, swift::DeclContext*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) + 141
16 swift 0x000000010ff1759e validateParameterType(swift::ParamDecl*, swift::DeclContext*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver&, swift::TypeChecker&) + 142
17 swift 0x000000010ff17417 swift::TypeChecker::typeCheckParameterList(swift::ParameterList*, swift::DeclContext*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver&) + 487
18 swift 0x000000010ff0df5a checkGenericFuncSignature(swift::TypeChecker&, swift::GenericSignatureBuilder*, swift::AbstractFunctionDecl*, swift::GenericTypeResolver&) + 266
19 swift 0x000000010ff0dd82 swift::TypeChecker::validateGenericFuncSignature(swift::AbstractFunctionDecl*) + 2178
20 swift 0x000000010fee2a57 swift::TypeChecker::validateDecl(swift::ValueDecl*) + 6263
21 swift 0x000000010fef5822 (anonymous namespace)::DeclChecker::visitFuncDecl(swift::FuncDecl*) + 82
22 swift 0x000000010fede254 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 180
23 swift 0x000000010fef5a7b (anonymous namespace)::DeclChecker::visitExtensionDecl(swift::ExtensionDecl*) + 491
24 swift 0x000000010fede283 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 227
25 swift 0x000000010fede18f swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) + 47
26 swift 0x000000010ff75dce swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int) + 1630
27 swift 0x000000010fbb7b10 swift::CompilerInstance::parseAndTypeCheckMainFile(swift::PersistentParserState&, swift::DelayedParsingCallbacks*, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>) + 416
28 swift 0x000000010fbb67d2 swift::CompilerInstance::parseAndCheckTypes(swift::CompilerInstance::ImplicitImports const&) + 626
29 swift 0x000000010fbb6132 swift::CompilerInstance::performSema() + 466
30 swift 0x000000010f049dff performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1887
31 swift 0x000000010f04868e swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3310
32 swift 0x000000010f006a63 main + 2051
33 libdyld.dylib 0x00007fff5997f115 start + 1
Stack dump:
0. Program arguments: /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2018-03-26-a.xctoolchain/usr/bin/swift -frontend -interpret test.swift -enable-objc-interop -sdk /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -module-name test

1. While type-checking extension of S<Int> at test.swift:4:1
2. While type-checking 'process(callback🙂' at test.swift:7:5
3. While validating 'process(callback🙂' at test.swift:7:5
4. While resolving type Callback at [test.swift:7:28 - line:7:28] RangeText="C"

By adjusting the code to not use the typealias, the code compiles without issue:

struct S<T> {}

extension S where T == Int {
    typealias Callback = (Bool) -> Void

    func process(callback: (Bool) -> Void) {   
    }
}