[analyzer] trackNullOrUndefValue: always track through parentheses and casts.

When trying to figure out where a null or undefined value came from,
parentheses and cast expressions are either completely irrelevant, or,
in the case of lvalue-to-rvale cast, straightforwardly lead us in the right
direction when we remove them.

There is a regression that causes a certain diagnostic to appear twice in the
path-notes.cpp test (changed to FIXME). It would be addressed in the next
commit.

Differential revision: https://reviews.llvm.org/D41254

llvm-svn: 321133