Expression eval lookup speedup by not returning methods in ManualDWARFIndex::GetFunctions

Summary:
This change is connected with
https://reviews.llvm.org/D69843

In large codebases, we sometimes see Module::FindFunctions (when called from
ClangExpressionDeclMap::FindExternalVisibleDecls) returning huge amounts of
functions.

In current fix I trying to return only function_fullnames from ManualDWARFIndex::GetFunctions when eFunctionNameTypeFull is passed as argument.

Reviewers: labath, jarin, aprantl

Reviewed By: labath

Subscribers: shafik, clayborg, teemperor, arphaman, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70846