[analyzer] Correctly model iteration through "nil" objects

Previously, iteration through nil objects which resulted from
objc-messages being set to nil were modeled incorrectly.

There are a couple of notes about this patch:

In principle, ExprEngineObjC might be left untouched IFF osx.loops
checker is enabled.
I however think that we should not do something
completely incorrect depending on what checkers are left on.
We should evaluate and potentially remove altogether the isConsumedExpr
performance heuristic, as it seems very fragile.

rdar://22205149

Differential Revision: https://reviews.llvm.org/D44178

llvm-svn: 326982