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

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `clang` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/45408

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-unified-tree) failure: build (failure) (timed out)
...
234.464 [92/8/7325] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/OpenMP/Decomposer.cpp.o
234.466 [92/7/7326] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/Support/ReductionProcessor.cpp.o
234.467 [92/6/7327] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/Support/Utils.cpp.o
234.468 [92/5/7328] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/SymbolMap.cpp.o
234.470 [92/4/7329] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/PFTBuilder.cpp.o
234.473 [92/3/7330] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/OpenMP/OpenMP.cpp.o
234.704 [91/3/7331] Linking CXX static library lib/libFortranLower.a
234.813 [90/3/7332] Linking CXX static library lib/libflangFrontend.a
237.729 [89/3/7333] Linking CXX executable bin/bbc
246.442 [89/2/7334] Building CXX object tools/clang/lib/ASTMatchers/Dynamic/CMakeFiles/obj.clangDynamicASTMatchers.dir/Registry.cpp.o
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1448.031408

```

</details>

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