replace clang LLVM_ENABLE_PLUGINS -> CLANG_PLUGIN_SUPPORT in tests Ensure CLANG_PLUGIN_SUPPORT is compatible with llvm_add_library. Fixes an issue noted in D111100. Differential Revision: https://reviews.llvm.org/D119199 GitOrigin-RevId: 76cad51ba700233d6e3492eddcbb466b6adbc2eb