[clang] Teach -fembed-bitcode option not to embed W_value Group

Summary:
-fembed-bitcode options doesn't embed warning options since they are
useless to code generation. Make sure it handles the W_value group and
not embed those options in the output.

Reviewers: zixuw, arphaman

Reviewed By: zixuw

Subscribers: jkorous, dexonsmith, ributzka, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83813