Add support to -Wa,-W in clang

Summary:
Currently clang does not support -Wa,-W, which suppresses warning
messages in GNU assembler. Add this option for gcc compatibility.
https://bugs.llvm.org/show_bug.cgi?id=43651. Reland with differential
information.

Reviewers: bcain

Reviewed By: bcain

Subscribers: george.burgess.iv, gbiv, llozano, manojgupta, nickdesaulniers, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68884

llvm-svn: 374834