This introduces `calling_convention_param_ids`, a single block that
consolidates all the information that was being used by consumers of
`param_refs` and `implicit_param_refs`, in a form that's easier to
produce and typically easier to consume.
See also [this Discord
discussion](https://discord.com/channels/655572317891461132/655578254970716160/1300545448909738125)
regarding the decision to keep the return slot last in the SemIR calling
convention, even though it goes first in the LLVM calling convention.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Also propagate the pattern IR along with the pattern-match IR, and use
it where appropriate.
Strictly speaking, some parts of the pattern-match IR are allocated
eagerly, while traversing the pattern's parse tree, but they still
aren't actually emitted until we traverse the associated pattern insts.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Geoff RomerandRichard Smith
