Check that the implicit cast from `id` used to construct the element
variable in an ObjC for-in statement is valid.
This check is included as part of a new `objc-cast` sanitizer, outside
of the main 'undefined' group, as (IIUC) the behavior it's checking for
is not technically UB.
The check can be extended to cover other kinds of invalid casts in ObjC.
Partially addresses: rdar://12903059, rdar://9542496
Differential Revision: https://reviews.llvm.org/D71491
Fangrui Song