NFC: DebugInfo: Reduce scope of DebugOffset to simplify code

This was being used as a sort of indirect out parameter from shouldDump
- seems simpler to use it as the actual result of the call. (this does
mean using a pointer to an Optional & actually using all 3 states (null,
None, and present) which is, admittedly, a tad subtle - but given the
limited scope, seems OK to me - open to discussion though, if others
feel strongly about it)

llvm-svn: 346691