Issue Description:

The Swift 4.0 compiler (2017-05-24-a) crashes when compiling the following code that works with the Swift 3.x compiler:

Reduced Repro Case:

class Base { }
class BaseSubclass: Base { }

func test() {
    class TestClass<T: Base> { } // Causes Swift 4.0 compiler crash
    let _ = TestClass<BaseSubclass>()
}

Steps to Reproduce:

1. Attempt to compile the code above with the Swift 4.0 snapshot specified.

Result:

1. Compilation fails with error:

Command failed due to signal: Abort trap: 6
...
Assertion failed: ((DWARFMangling || type->isCanonical()) && "expecting canonical types when not mangling for the debugger"), function appendType, file /Users/buildnode/jenkins/workspace/oss-swift-4.0-package-osx/swift/lib/AST/ASTMangler.cpp, line 581.

Result with Swift 3.x:

1. Compilation succeeds.

Swift 4.0 workaround:
Moving the

class TestClass<T: Base> { }

line outside of the test() function resolves the crash.