This combines the libc++ and libstdc++ test cases. The main difference
was that the libstdcpp tests had additional tests for
references/pointers to std::string. I moved those over.
The libstdc++ formatters don't support `std::u16string`/`std::u32string`
yet, so I extracted those into XFAILed test cases.
There were also two test assertions that failed for libstdc++:
1. libstdc++ doesn't obey the capped/uncapped summary options
2. When a summary isn't available for a std::string, libc++ would print
"Summary Unavailable", whereas libstdc++ just prints "((null))". This
may be better suited for the STL-specific subdirectories, but left it
here for now.
I put those in separate XFAILed test-cases.
Split out from https://github.com/llvm/llvm-project/pull/146740
1f28da60d5