[clang] [clang] Extend -Wunused-but-set-variable to static globals (PR #178342)

[Yanzuo Liu via cfe-commits]

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zwuis wrote:

Move changes in this file to here to put code handling similar things (diagnosing unused things) together.

Run this logic only if this diagnostic is enabled like line 1592.

https://github.com/llvm/llvm-project/pull/178342