When converting a block pointer to an Objective-C pointer type, extend

When converting a block pointer to an Objective-C pointer type, extend
the lifetime of the block by copying it to the heap, or else we'll get
a dangling reference because the code working with the non-block-typed
object will not know it needs to copy.

There is some danger here, e.g. with assigning a block literal to an
unsafe variable, but, well, it's an unsafe variable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139451 91177308-0d34-0410-b5e6-96231b3b80d8