[analyzer] Refine the diagnostics in the nullability checker to differentiate between nil and null

This is a big deal for ObjC, where nullability annotations are extensively
used. I've also changed "Null" -> "null" and removed "is" as this is the
pattern that Sema is using.

Differential Revision: https://reviews.llvm.org/D27600

llvm-svn: 289885