[DebugInfo][InstrRef] Pre-land on-by-default-for-x86 changes

Over in D114631 and [0] there's a plan for turning instruction referencing
on by default for x86. This patch adds / removes all the relevant bits of
code, with the aim that the final patch is extremely small, for an easy
revert. It should just be a condition in CommandFlags.cpp and removing the
XFail on instr-ref-flag.ll.

[0] https://lists.llvm.org/pipermail/llvm-dev/2021-November/153653.html