[ARM] Fix prologue/epilogue for pacbti-m leaf functions

R12 is callee-saved in functions with pacbti-m enabled, but this is
done in assignCalleeSavedSpillSlots, meaning that in
determineCalleeSaves we have to manually set CanEliminateFrame.

This fixes a bug where in leaf functions with no other callee-saved
registers the aut instruction wouldn't be emitted and stack offsets
of arguments passed on the stack would be incorrect.

Differential Revision: https://reviews.llvm.org/D157865