swift234

Code completion: make sure to always emit a diagnostic so that ASTContext is
marked as erroneous

This is important so that other parts of the compiler (e.g., AST verifier) are
less strict about AST invariants.

Alternatively, we could make sure to emit a diagnostic in the parser when we
consume the code completion token, but chasing every place where we do it now
(and remembering to do this in future) does not scale well.

Fixes the crash in rdar://problem/16100914


Swift SVN r14865