Rejecting the mutable specifier on a freestanding type declaration, instead of...

Rejecting the mutable specifier on a freestanding type declaration, instead of supporting it as a "extension" (which serves no purpose). Un-XFAILing the test for mutable specifiers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209635 91177308-0d34-0410-b5e6-96231b3b80d8