objective-C arc: in copy helper function for 
__strong __block variables, perform objc_storeStrong on
source and destination instead of direct move. This 
is done with -O0 and to improve some analysis.
// rdar://12530881


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171555 91177308-0d34-0410-b5e6-96231b3b80d8