[lsan][darwin] Unmask camouflaged class_rw_t pointers

[lsan][darwin] Unmask camouflaged class_rw_t pointers

Detailed motivation here: https://docs.google.com/document/d/1xUNo5ovPKJMYxitiHUQVRxGI3iUmspI51Jm4w8puMwo

check-asan (with LSAN enabled) and check-lsan are currently broken on recent macOS versions, due to pervasive false positives. Whenever the Objective-C runtime realizes a class, it allocates data for it, then stores that data with flags in the low bits. This means LSAN can not recognize it as a pointer while scanning.

This change checks every potential pointer on Apple platforms, and if the high bit is set, attempts to extract a pointer by masking out the high bit and flags. This is ugly, but it's also the best approach I could think of (see doc above); very open to other suggestions.

Differential Revision: https://reviews.llvm.org/D133126