GCC compatibility: Ignore -finput_charset=UTF-8 argument. It is the default in…

Description

GCC compatibility: Ignore -finput_charset=UTF-8 argument. It is the default in Clang.
Currently, we fail with an error.

Reviewers: rafael

Reviewed By: rafael

Subscribers: rnk, cfe-commits

Differential Revision: http://reviews.llvm.org/D4347