[lldb][DWARFUnit] Implement PeekDIEName query (#78486)

This allows us to query the AT_Name of a DIE without parsing the entire
CU.

Part of the ongoing effort to support IDX_Parent in accelerator tables
[1].

[1]:
https://discourse.llvm.org/t/rfc-improve-dwarf-5-debug-names-type-lookup-parsing-speed/74151/44