Implement C++ name lookup for instance variables of Objective-C classes

from an instance method. Previously, we were following the Objective-C
name lookup rules for ivars, which are of course completely different
from and incompatible with the Objective-C++ rules.

For the record, the Objective-C++ rules are the sane ones.

This is another part of <rdar://problem/7660386>.

llvm-svn: 96677