Update the CFA to use $sp when $fp is restored on arm64

In UnwindAssemblyInstEmulation we correctly recognize when a LDP
restores the fp & lr in an epilogue, and mark them as having the
caller's contents now, but we don't update the CFA register rule
at that point to indicate that the CFA is now calculated in terms
of $sp.  This doesn't impact the backtrace because the register
contents are all <same> now, but it can confuse the stepper when
the StackID changes mid-epilogue.

Differential Revision: https://reviews.llvm.org/D124492
rdar://92064415