[SymbolFileDWARF][NFC] Remove duplicated code checking for type tags (#74773)

There was duplicated (and complex) code querying whether tags were
type-like tags (i.e. class or struct); this has been factored out into a
helper function.

There was also a comment about not comparing identical DIEs without ever
performing that check; this comment has been removed. It was likely a
result of copy paste from another function in this same file which
actually does that check.