[lldb][DWARFIndex] Use IDX_parent to implement GetFullyQualifiedType query (#79932)

This commit changes DebugNamesDWARFIndex so that it now overrides
`GetFullyQualifiedType` and attempts to use DW_IDX_parent, when
available, to speed up such queries. When this type of information is
not available, the base-class implementation is used.

With this commit, we now achieve the 4x speedups reported in [1].

[1]:
https://discourse.llvm.org/t/rfc-improve-dwarf-5-debug-names-type-lookup-parsing-speed/74151/38