[Darwin] Improve ASan diagnostics on arm64e with pointer auth

When reporting diagnostics from ASan's (and other sanitizer's) signal
handlers we should strip the "invalid signature" bit before printing
addresses.  This makes the report less confusing and let's the user
focus on the real issue.

rdar://62615826

Reviewed By: kubamracek, delcypher

Differential Revision: https://reviews.llvm.org/D79132