[analyzer] Fix r168019 to work with unpruned paths as well.

This is the case where the analyzer tries to print out source locations
for code within a synthesized function body, which of course does not have
a valid source location. The previous fix attempted to do this during
diagnostic path pruning, but some diagnostics have pruning disabled, and
so any diagnostic with a path that goes through a synthesized body will
either hit an assertion or emit invalid output.

<rdar://problem/12657843> (again)

llvm-svn: 169631