[lldb/DWARF] Refactor DWARFDIE::Get{Decl,TypeLookup}Context (#93291)

After a bug (the bug is that the functions don't handle DW_AT_signature,
aka type units) led me to one of these similar-but-different functions,
I started to realize that most of the differences between these two
functions are actually bugs.

As a first step towards merging them, this patch rewrites both of them
to follow the same pattern, while preserving all of their differences.
The main change is that GetTypeLookupContext now also uses a `seen` list
to avoid reference loops (currently that's not necessary because the
function strictly follows parent links, but that will change with
DW_AT_signatures).

I've also optimized both functions to avoid recursion by starting contruction
with the deepest scope first (and then reversing it).