[ObjCRuntime] Add support for obfuscation in tagged pointers.

This is the default in MacOS Mojave. No testcases, as basically
we have a lot of coverage (and the testsuite fails quite a bit
without this change in Beta 3).

Thanks to Fred Riss for helping me with this patch (fixing
bugs/nondeterminism).

<rdar://problem/38305553>

llvm-svn: 336607