[lldb] Move DWARFDeclContext functions from DWARFDebugInfoEntry to DW… (#95227)

…ARFDIE

This puts them closer to the other two functions doing something very
similar. I've tried to stick to the original logic of the functions as
much as possible, though I did apply some easy simplifications.

The changes in DWARFDeclContext.h are there to make the unit tests
produce more useful error messages.