fd997d86f0
This combines the libc++ and libstdc++ test cases. The main difference was that the libstdcpp tests had some tuple indexing tests that libc++ didn't have. The libstdc++ formatter didn't support size summaries for std::tuple. So I added a `ContainerSizeSummaryProvider` for it (like we do for libc++). Additionally, the synthetic frontend would only apply to non-empty tuples, so I adjusted the regex to match empty ones too. We do this for libc++ already. Split out from https://github.com/llvm/llvm-project/pull/146740