[LLDB][Formatters] Re-enable std::function formatter with fixes to improve...

[LLDB][Formatters] Re-enable std::function formatter with fixes to improve non-cached lookup performance

Performance issues lead to the libc++ std::function formatter to be disabled. We addressed some of those performance issues by adding caching see D67111
This PR fixes the first lookup performance by not using FindSymbolsMatchingRegExAndType(...) and instead finding the compilation unit the std::function wrapped callable should be in and then searching for the callable directly in the CU.

Differential Revision: https://reviews.llvm.org/D69913