[objc_direct] Allow for direct messages be sent to `self` when it is a Class

Sending a message to `self` when it is const and within a class method
is safe because we know that `self` is the Class itself.

We can only relax this warning in ARC.

Signed-off-by: Pierre Habouzit <phabouzit@apple.com>
Radar-Id: rdar://problem/58581965
Differential Revision: https://reviews.llvm.org/D72747