[Bash-autocompletion] Add support for -W<warning> and -Wno<warning>

Summary:
`-W[tab]` will autocomplete warnings defined in this link:
https://clang.llvm.org/docs/DiagnosticsReference.html#wweak-vtables

Differential Revision: https://reviews.llvm.org/D35447

llvm-svn: 308139