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Issue Description:

Compiling the attached segmentionfault.swift causes the compiler to crash.

$ swiftc --version
Apple Swift version 5.1.2 (swiftlang-1100.0.278 clang-1100.0.33.9)
Target: x86_64-apple-darwin18.7.0

$ swiftc segmentionfault.swift
Stack dump:``0. Program arguments: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -c -primary-file segmentionfault.swift -target x86_64-apple-darwin18.7.0 -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -color-diagnostics -module-name segmentionfault -o /var/folders/0s/0f_f8b0j0zn5t5r6yv5b6lth0000gp/T/segmentionfault-9efcb6.o ``1. While emitting SIL for 'zipLongestStrings(:🙂' (at segmentionfault.swift:12:1)``2. While silgen emitFunction SIL function "@$s15segmentionfault17zipLongestStringsyySaySSG_SbAA17CharacterSequenceVXEtF".{{ for 'zipLongestStrings(*:*🙂' (at segmentionfault.swift:12:1)}}0 swift 0x00000001061a5a13 PrintStackTraceSignalHandler(void*) + 51``1 swift 0x00000001061a51e6 SignalHandler(int) + 358``2 libsystem_platform.dylib 0x00007fff6749ab5d _sigtramp + 29``3 libsystem_platform.dylib 0x00007fff0394c506 _sigtramp + 2622167494``4 swift 0x0000000102298017 swift::Lowering::SILGenFunction::emitRValueForNonMemberVarDecl(swift::SILLocation, swift::VarDecl*, swift::CanType, swift::AccessSemantics, swift::Lowering::SGFContext) + 1767``5 swift 0x000000010225dc34 swift::Lowering::SILGenFunction::emitRValueForDecl(swift::SILLocation, swift::ConcreteDeclRef, swift::Type, swift::AccessSemantics, swift::Lowering::SGFContext) + 548``6 swift 0x0000000102250c5a swift::ASTVisitor<(anonymous namespace)::RValueEmitter, swift::Lowering::RValue, void, void, void, void, void, swift::Lowering::SGFContext>::visit(swift::Expr*, swift::Lowering::SGFContext) + 7002``7 swift 0x000000010224fcd6 swift::ASTVisitor<(anonymous namespace)::RValueEmitter, swift::Lowering::RValue, void, void, void, void, void, swift::Lowering::SGFContext>::visit(swift::Expr*, swift::Lowering::SGFContext) + 3030``8 swift 0x000000010228e295 swift::Lowering::SILGenFunction::emitAssignToLValue(swift::SILLocation, swift::Lowering::ArgumentSource&&, swift::Lowering::LValue&&) + 709``9 swift 0x000000010225704d swift::ASTVisitor<(anonymous namespace)::RValueEmitter, swift::Lowering::RValue, void, void, void, void, void, swift::Lowering::SGFContext>::visit(swift::Expr*, swift::Lowering::SGFContext) + 32589``10 swift 0x0000000102264f6f swift::Lowering::SILGenFunction::emitIgnoredExpr(swift::Expr*) + 799``11 swift 0x00000001022c4cf3 swift::ASTVisitor<(anonymous namespace)::StmtEmitter, void, void, void, void, void, void>::visit(swift::Stmt*) + 3139``12 swift 0x00000001022c5d8e swift::ASTVisitor<(anonymous namespace)::StmtEmitter, void, void, void, void, void, void>::visit(swift::Stmt*) + 7390``13 swift 0x00000001022c4c9b swift::ASTVisitor<(anonymous namespace)::StmtEmitter, void, void, void, void, void, void>::visit(swift::Stmt*) + 3051``14 swift 0x000000010227ab5a swift::Lowering::SILGenFunction::emitFunction(swift::FuncDecl*) + 426``15 swift 0x00000001021dfd24 swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) + 980``16 swift 0x00000001021ea806 swift::Lowering::SILGenModule::emitSourceFile(swift::SourceFile*) + 1238``17 swift 0x00000001021ec266 swift::SILModule::constructSIL(swift::ModuleDecl*, swift::SILOptions&, swift::FileUnit*) + 1238``18 swift 0x0000000101defda4 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 28340``19 swift 0x0000000101de56e4 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 6820``20 swift 0x0000000101d72be3 main + 1219``21 libdyld.dylib 0x00007fff672af3d5 start + 1``22 libdyld.dylib 0x000000000000000f start + 2564099131``<unknown>:0: error: unable to execute command: Segmentation fault: 11``<unknown>:0: error: compile command failed due to signal 11 (use -v to see invocation)