DebugInfo: Migrate callers from getAsCString to dwarf::toString

This makes a bunch of these call sites independent of a follow-up change
I'm making to have getAsCString return Expected<const char*> for more
descriptive error messages so that the failures there can be
communicated up to DWARFVerifier (or other callers who want to provide
more verbose diagnostics) so DWARFVerifier doesn't have to re-implement
the string lookup logic and error checking.