[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.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179736 91177308-0d34-0410-b5e6-96231b3b80d8
Update the -Wparentheses tests to check that fix-its are in the correct place.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179740 91177308-0d34-0410-b5e6-96231b3b80d8