[LLDB] Adding caching to libc++ std::function formatter for lookups that require scanning symbols

Performance issues lead to the libc++ std::function formatter to be disabled.
This change is the first of two changes that should address the performance issues and allow us to enable the formatter again.
In some cases we end up scanning the symbol table for the callable wrapped by std::function for those cases we will now cache the results and used the cache in subsequent look-ups. This still leaves a large cost for the initial lookup which will be addressed in the next change.

Differential Revision: https://reviews.llvm.org/D67111