[analyzer] Teach trackNullOrUndefValue() about class property accessors.

Teach trackNullOrUndefValue() how to properly look through PseudoObjectExprs
to find the underlying semantic method call for property getters. This fixes a
crash when looking through class property getters that I introduced in r265839.

rdar://problem/26796666

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273340 91177308-0d34-0410-b5e6-96231b3b80d8