GCC compatibility: Ignore -fexec-charset=UTF-8 argument. It is the default in Clang. Reject other values.

Summary:
Just like with -finput-charset=UTF-8 in review http://reviews.llvm.org/D4347, I think we should just ignore it when UTF-8 is provided.


Reviewers: rnk, rafael

Reviewed By: rafael

Subscribers: rafael, cfe-commits

Differential Revision: http://reviews.llvm.org/D4841