[lldb][test] Test all libcxxabi demangler test-cases against TrackingOutputBuffer

To test the infrastructure added in #131836 in would be nice to confirm that we can reconstruct all kinds of demangled names. The libcxxabi test-suite already has all those test-cases.

This patch copies those test-cases (taken from `libcxxabi/test/test_demangle.pass.cpp`), reconstructs the name like LLDB would when showing backtraces, and confirms that all demangled names can be fully reconstructed.

Two open questions:
1. Do we really want a copy of all those test-cases in LLDB? It's
   unlikely to be kept in sync with the demangler test-suite.
2. Do we want to turn the `GetDemangledBasename`/`GetDemangledScope`/etc. into public APIs (e.g., on `TrackingOutputBuffer`) so that we can use the exact same method of extraction in the tests?

Author: Michael137