[clang] refactor -Wa,-W test cases.

Remove REQUIRES and only keep the clang driver tests, since the
assembler are already tested with -Wa,--no-warn. This way we could run
the test on non-linux platforms and catch breaks on them.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374932 91177308-0d34-0410-b5e6-96231b3b80d8