Objective-C IRGen. Use llvm::WeakVH
for caching couple of global symbols used
for generation of CF/NS string meta-data
so they are not released prematuely in certain
corner cases. // rdar:// 13598026.
Reviewed by John M.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179599 91177308-0d34-0410-b5e6-96231b3b80d8