Debugger: DwarfFile: Clean up type units on delete.
Debugger: Add convenience function to BaseUnit...

...to query if a given global offset is within its range.
Debugger: implement support for section-global references.

- Add DwarfFile::_GetContainingCompilationUnit() to request a binary
search for the compilation unit containing the passed in global offset.
- DwarfFile::_ResolveReference(): handle global references by first
attempting to find a containing compilation unit, and then seeing if we
can find an entry for the corresponding relative offset within it.