[Sanitizer] Strip PAC from PC in StackTrace::GetNextInstructionPc()

In TSan, we use the a function reference (`__tsan_stack_initialization`)
in a call to `StackTrace::GetNextInstructionPc(uptr pc)`.  We sign
function pointers, so we need to strip the signature from this function
pointer.

Caused by: https://reviews.llvm.org/D111147

Radar-Id: rdar://problem/83940546