[DebugInfo] Rename section identifiers which are deprecated in DWARFv5. NFC.

This is a preparation for an upcoming patch which adds support for
DWARFv5 unit index sections. The patch adds tag "_EXT_" to identifiers
which reference sections that are deprecated in the DWARFv5 standard.
See D75929 for the discussion.

Differential Revision: https://reviews.llvm.org/D77141