build fix for SBInstruction.

oops!  I cherry-picked  rL374820 thinking it was completely
independent of D68737, but it wasn't.  It makes an incidental
use of SBFile::GetFile, which is introduced there, so I broke the
build.

The docs say you can commit without review for "obvious".   I think
this qualifies.   If this kind of fix isn't considered obvious, let
me know and I'll revert instead.

Fixes: rL374820
llvm-svn: 374825