[lldb][NFCish] Move DWARFDebugInfoEntry::GetQualifiedName() into DWARFASTParserClang

In D134378, we'll need the clang AST to be able to construct the qualified in some cases.

This makes logging in one place slightly less informative.

Reviewed By: dblaikie, Michael137

Differential Revision: https://reviews.llvm.org/D135979