Issue Description:

In apple/swift#15996 I'm adding a check to the Swift AST verifier that open is not used on declarations where it doesn't make sense (like a typealias). However, LLDB's unilaterally splatting open and public on things defined in the REPL to make sure they can be accessed later. That's a fine thing to do, but the condition where it was doing so wasn't quite right. I'm fixing that, but I couldn't write a real test for it because the AST verifier runs as part of Swift type checking, and is not run again after LLDB does its manipulation.

LLDB should call swift::verify(SourceFile &SF) (which is a no-op in release builds) after it does any AST manipulation, to make sure that the resulting AST still matches what the later phases of the compiler will expect.