[lsan][Darwin] Unconditionally strip high bits from potential pointers

The method cache stashes a mask in the high bits under some circumstances:
https://github.com/apple-oss-distributions/objc4/blob/689525d556eb3dee1ffb700423bccf5ecc501dbf/runtime/objc-cache.mm#L589

I'm hitting this now on macOS 13.4 arm64, so we can no longer rely on OBJC_FAST_IS_RW to identify potential pointers that need to be transformed

Differential Revision: https://reviews.llvm.org/D153471