[lldb/DWARF] Only match mangled name in full-name function lookup (with accelerators)

Summary:
In the spirit of https://reviews.llvm.org/D70846, we only return functions with matching mangled name from Apple/DebugNamesDWARFIndex::GetFunction if eFunctionNameTypeFull is requested.

This speeds up lookup in the presence of large amount of class methods of the same name (a typical examples would be constructors of templates with many instantiations or overloaded operators).

Reviewers: labath

Reviewed By: labath

Subscribers: aprantl, arphaman, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D73191