Add self-comparison warnings for fields.

This expands very slightly what -Wtautological-compare considers to be
tautological to include implicit accesses to C++ fields and ObjC ivars.
I don't want to turn this into a full expression-identity check, but
these additions seem pretty well-contained, and maintain the theme
of checking for "x == x".

<rdar://problem/14431127>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190118 91177308-0d34-0410-b5e6-96231b3b80d8