Add a fatal error for debug builds when disagreement about stepping

On one CI bot we're seeing a failure where the kernel reports that
we have completed an instruction step (via a mach exception) and
lldb doesn't think the thread was doing an instruction step.  It
takes the conservative approach of stopping at this point, breaking
tests.

This patch adds an llvm fatal error for debug builds where it will
log the state of the thread and the AArch64 ESR, to confirm what
the hardware reported as the exception so we can double check the
kernel's interpretation.

I'll change this to an lldbassert without the runtime details in
the string once we have an idea what is happening.  the hope is
that this will get hit on the CI bot soon.

Differential Revision: https://reviews.llvm.org/D153079