[Parser] Fix assertion-on-invalid for unexpected typename.

In `ParseDeclarationSpecifiers` for the code

    class A typename A;

we were able to annotate token `kw_typename` because it refers to
existing type. But later during processing token `annot_typename` we
failed to `SetTypeSpecType` and exited switch statement leaving
annotation token unconsumed. The code after the switch statement failed
because it didn't expect a special token.

The fix is not to assume that switch statement consumes all special
tokens and consume any token, not just non-special.

rdar://problem/37099386

Reviewers: rsmith, arphaman

Reviewed By: rsmith

Subscribers: jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D44449

llvm-svn: 329735