[analyzer] pr38838, pr39976: Fix crash on diagnosing before implicit destructor.

[analyzer] pr38838, pr39976: Fix crash on diagnosing before implicit destructor.

We need to be able to emit the diagnostic at PreImplicitCall,
and the patch implements this functionality.

However, for now the need for emitting such diagnostics is not all that great:
it is only necessary to not crash when emitting a false positive due to an
unrelated issue of having dead symbol collection not working properly.

Coming up with a non-false-positive test seems impossible with the current
set of checkers, though it is likely to be needed for good things as well
in the future.

Differential Revision: https://reviews.llvm.org/D56042

rdar://problem/46911462

llvm-svn: 350907