[Debuginfo][DWARF][NFC] Add paired methods working with DWARFDebugInfoEntry.

This review is extracted from D96035.

DWARF Debuginfo classes have two representations for DIEs: DWARFDebugInfoEntry
(short) and DWARFDie(extended). Depending on the task, it might be more convenient
to use DWARFDebugInfoEntry or/and DWARFDie. DWARFUnit class already has methods
working with DWARFDie and DWARFDebugInfoEntry. This patch adds more
methods working with DWARFDebugInfoEntry to have paired functionality.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D126059