[objc-arc] Turn off the objc_retainBlock -> objc_retain optimization.

The reason that I am turning off this optimization is that there is an
additional case where a block can escape that has come up. Specifically, this
occurs when a block is used in a scope outside of its current scope.

This can cause a captured retainable object pointer whose life is preserved by
the objc_retainBlock to be deallocated before the block is invoked.

An example of the code needed to trigger the bug is:

----
\#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
  void (^somethingToDoLater)();

  {
    NSObject *obj = [NSObject new];

    somethingToDoLater = ^{
      [obj self]; // Crashes here
    };
  }

  NSLog(@"test.");

  somethingToDoLater();
  return 0;
}
----

In the next commit, I remove all the dead code that results from this.

Once I put in the fixing commit I will bring back the tests that I deleted in
this commit.

rdar://14802782.
rdar://14868830.

llvm-svn: 189869