DebugInfo: Fix cases where location failed to be updated after r225000

The optimization (that appears to have been here since the earliest
implementation (r50848) & has become more complicated over the years) to
avoid recreating the debugloc if it would be the same was out of date
because ApplyDebugLocation was not re-updating the CurLoc/PrevLoc. This
optimization doesn't look terribly beneficial/necessary, so I'm removing
it - if it turns up in benchmarks, I'm happy to reconsider/reimplement
this with justification, but for now it just seems to add
complexity/problems.

llvm-svn: 225083