Objective-C ARC. Blocks that strongly capture themselves

Objective-C ARC. Blocks that strongly capture themselves
to call themselves will get the warning:
"Capturing <itself> strongly in this block is likely to
lead to a retain cycle". Cut down on the amount of noise
by noticing that user at some point sets the captured variable
to null in order to release it (and break the cycle).
// rdar://16944538

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210823 91177308-0d34-0410-b5e6-96231b3b80d8