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>

llvm-svn: 190118