-Warc-retain-cycles: look through [^{...} copy] and Block_copy(^{...})

Retain cycles happen in the case where a block is persisted past its
life on the stack, and the way that occurs is by copying the block.
We should thus look through any explicit copies we see.

Note that Block_copy is actually a type-safe wrapper for _Block_copy,
which does all the real work.

<rdar://problem/12219663>

llvm-svn: 164039