[analyzer] Pointers-to-members are (currently) Locs, not NonLocs.

While we don't model pointers-to-members besides "null" and "non-null",
we were using Loc symbols for valid pointers and NonLoc integers for the
null case. This hit the assert committed in r185401.

Fixed by using a true (Loc) null for null member pointers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185444 91177308-0d34-0410-b5e6-96231b3b80d8