[clang] Clang: Add warning flag for storage class specifiers on explicit specializations (PR #96699)

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https://github.com/alexfh commented:

Should we add a test for this flag? Something around existing tests in clang/test/CXX/temp/temp.spec/temp.expl.spec/p2-20.cpp and clang/test/CXX/dcl.dcl/dcl.spec/dcl.stc/p1.cpp.

https://github.com/llvm/llvm-project/pull/96699