[clang] [compiler-rt] [clang] Add -Wunused-but-set-global; fix compiler-rt findings (PR #188291)

[Hans Wennborg via cfe-commits]

zmodem wrote:

> I'm not sure if a separate flag is a right approach.

Adding separate flags for new or expanded warnings is, if not written policy, at least a long established practice.

In this case it seems a good fit since the scope (globals) is clear.

https://github.com/llvm/llvm-project/pull/188291