As we approach the state where support for debug intrinsics is dropping and
we print and use debug records by default, the documentation should be updated
to refer to debug records as the primary debug info representation, with
debug intrinsics being relegated to an optional alternative.

This patch performs a few updates:
- Replace references to intrinsics with references to records across all
the documentation.
- Replace intrinsics with records in code examples.
- Move debug records prior to debug intrinsics in the
SourceLevelDebugging document, and change text to refer to them as the
primary representation.
- Add release notes describing the change.

This adds code blocks and inline code formatting to improve the readability of
the instruction referencing doc.

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D126767

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>