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
fd997d86f0