[clang] Silence a false positive GCC -Wunused-but-set-parameter warning with constexpr

This fixes the following warning:

    In file included from ../tools/clang/lib/Tooling/Transformer/Transformer.cpp:9:
    ../tools/clang/include/clang/Tooling/Transformer/Transformer.h: In instantiation of ‘llvm::Error clang::tooling::detail::populateMetadata(const clang::transformer::RewriteRuleWith<MetadataT>&, size_t, const clang::ast_matchers::MatchFinder::MatchResult&, clang::tooling::TransformerResult<T>&) [with T = void; size_t = long unsigned int]’:
    ../tools/clang/include/clang/Tooling/Transformer/Transformer.h:179:34:   required from ‘void clang::tooling::detail::WithMetadataImpl<T>::onMatchImpl(const clang::ast_matchers::MatchFinder::MatchResult&) [with T = void]’
    ../tools/clang/include/clang/Tooling/Transformer/Transformer.h:156:8:   required from here
    ../tools/clang/include/clang/Tooling/Transformer/Transformer.h:120:25: warning: parameter ‘SelectedCase’ set but not used [-Wunused-bu...