[libcxx][test] Avoid warnings about unused variables and typedefs if `_LIBCPP_VERSION` is not defined#155679

philnik777 merged 3 commits intollvm:mainllvm/llvm-project:mainfrom

AlexGuteniev:flat-tests-unused-things-non-libcxxAlexGuteniev/llvm-project:flat-tests-unused-things-non-libcxxCopy head branch name to clipboard