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

[mstorsjo]

Summary

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-but-set-parameter]
  120 |                  size_t SelectedCase,
      |                  ~~~~~~~^~~~~~~~~~~~

The issue is fixed in GCC 10 and later, but this silences the noisy
warning in older versions. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85827
for more details about the bug.

[bkramer]

thanks

clang/include/clang/Tooling/Transformer/Transformer.h

 /// Runs the metadata generator on \c Rule and stuffs it into \c Result.
 /// @{
 template <typename T>
 llvm::Error
 populateMetadata(const transformer::RewriteRuleWith<T> &Rule,
                  size_t SelectedCase,
                  const ast_matchers::MatchFinder::MatchResult &Match,
                  TransformerResult<T> &Result) {
+  // Silence a false positive GCC -Wunused-but-set-parameter warning in constexpr
+  // cases, by marking SelectedCase as used. See
+  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85827 for details. The issue is
+  // fixed in GCC 10.
+  (void)SelectedCase;
   if constexpr (!std::is_void_v<T>) {
     auto Metadata = Rule.Metadata[SelectedCase]->eval(Match);
     if (!Metadata)
       return Metadata.takeError();
     Result.Metadata = std::move(*Metadata);
   }
   return llvm::Error::success();
 }