[DebugInfo] Turn instruction referencing on by default for x86

This patch is designed to be reverted -- it activates a reasonably large
block of new-ish code, so some turbulence is likely.

Instruction referencing is best summarised, and it being on-by-default,
is discussed here:

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

Differential Revision: https://reviews.llvm.org/D114631