[DebugInfo][NFC] Add instr-ref documentation, migration guide

This used to be D102158, but all the code it describes got re-written, so I
figured I'd take another shot at documenting the new instruction referencing
variable locations, this time from a higher level. Happily there's no longer any
need to describe LiveDebugValues in any detail seeing how it's all SSA-based
now.

Probably the most important part is the explanation of what targets need to do
to support instruction referencing. The list is small, mostly because there's
nothing especially complicated that targets need to do: just instrument their
target-specific optimisations and implement the stack spill/restore recognition
target hooks.

This is a small amount of text (which is a virtue), I'm extremely happy to
expand on anything.

Differential Revision: https://reviews.llvm.org/D113586

Co-authored-by: Jeremy Morse <jeremy.morse@sony.com>

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