[analyzer] Allow ObjC ivar lvalues where the base is nil.

By analogy with C structs, this seems to be legal, if probably discouraged.
It's only if the ivar is read from or written to that there's a problem.
Running a program that gets the "address" of an instance variable does in
fact return the offset when the base "object" is nil.

This isn't a full revert because r164442 includes some diagnostic tweaks
as well; those have been kept.

This partially reverts r164442 / 08965091770c9b276c238bac2f716eaa4da2dca4.

llvm-svn: 164960