Code and comment cleanups [NFC]

Changes:
- update comments to detail the info can come from .debug_info or .debug_types
- Rename "debug_info_data" to "data" now that we can get data from .debug_info or .debug_types.
- Also call DWARFDebugInfoEntry::GetAbbreviationDeclarationPtr(...) instead of manually grabbing abbreviation.

llvm-svn: 362116