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

[Devon Loehr via cfe-commits]

DKLoehr wrote:

Adding a new sub-flag is something we've done in the past to help migrations. We've started seeing this fire in many different places in chromium; we want to be able to disable it while we fix the code, without losing coverage of unused locals. 

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