[analyzer] Explicitly disallow mixed Loc-NonLoc comparisons.

The one bit of code that was using this is gone, and neither C nor C++
actually allows this. Add an assertion and remove dead code.

Found by Matthew Dempsky!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185401 91177308-0d34-0410-b5e6-96231b3b80d8