Warn if a __weak variable is initialized with an Objective-C object literal.

Such variables may immediately become nil or may have unpredictable
behavior.

Fixes <rdar://problem/12569201>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170763 91177308-0d34-0410-b5e6-96231b3b80d8