[ObjC] Pop all cleanups created in EmitObjCForCollectionStmt before

exiting the for-in loop.

This commit fixes a bug where EmitObjCForCollectionStmt didn't pop
cleanups for captures.

For example, in the following for-in loop, a block which captures self
is passed to foo1:

for (id x in [self foo1:^{ use(self); }]) {
  use(x);
  break;
}

Previously, the code in EmitObjCForCollectionStmt wouldn't pop the
cleanup for the captured self before exiting the loop, which caused
code-gen to generate an IR in which objc_release was called twice on
the captured self.

This commit fixes the bug by entering a RunCleanupsScope before the
loop condition is evaluated and forcing its cleanup before exiting the
loop.

rdar://problem/16865751

Differential Revision: http://reviews.llvm.org/D18618

llvm-svn: 266147