[analyzer] Tweak getDerefExpr more to track DeclRefExprs to references.

In the committed example, we now see a note that tells us when the pointer
was assumed to be null.

This is the only case in which getDerefExpr returned null (failed to get
the dereferenced expr) throughout our regression tests. (There were multiple
occurrences of this one.)

llvm-svn: 179736