Add support for parsing type unit entries in .debug_names. (#72952)

This is a follow up patch after .debug_names can now emit local type
unit entries when we compile with type units + DWARF5 + .debug_names.
The pull request that added this functionality was:

https://github.com/llvm/llvm-project/pull/70515

This patch makes sure that the DebugNamesDWARFIndex in LLDB will not
manually need to parse type units if they have a valid index. It also
fixes the index to be able to correctly extract name entries that
reference type unit DIEs. Added a test to verify things work as
expected.