[LLDB] On AArch64, reconfigure register context first (#70742)

On an SVE/SME the register context configuration may change after the
inferior process has executed. This was handled via
https://reviews.llvm.org/D159504 but it is reconfiguring and clearing
the register context after we've parsed any expedited reigster values
from the stop reply packet. That results in lldb having to read each
register value one at a time while at that stop location, which will be
a performance problem on non-local debug setups.

The configuration & clearing needs to happen first. Also, update the
names of the local variables for a little clarity.