[lldb] Fix RichManglingContext::FromCxxMethodName() leak

`RichManglingContext::FromCxxMethodName` allocates a m_cxx_method_parser, but never deletes it.

This fixes a `-DLLVM_USE_SANITIZER=Leaks` failure.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D100795