[Debuginfo][DWARF][NFC] Refactor DwarfStringPoolEntryRef.

This review is extracted from D96035.

This patch adds possibility to keep not only DwarfStringPoolEntry, but also
pointer to it. The DwarfStringPoolEntryRef keeps reference to the string map entry.
String map keeps string data and corresponding DwarfStringPoolEntry
info. Not all string map entries may be included into the result,
and then not all string entries should have DwarfStringPoolEntry
info. Currently StringMap keeps DwarfStringPoolEntry for all entries.
It leads to extra memory usage. This patch allows to keep
DwarfStringPoolEntry info only for entries which really need it.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D126883