Rework a few things about how we emit ObjC's for enumeration statement.

In particular, the iteration variable (if present) should be created and
destroyed in a narrow span around the loop body, and the body should
be emitted in a cleanup scope in case it's not a compound statement.
Otherwise, rename a few variables and use phis instead of temporary
variables for the index and buffer count.

llvm-svn: 122988