Restructure return declaration handling (#7076)

- A function with a return declaration always has exactly one
`ReturnSlotPattern`, representing the whole return declaration (whereas
previously that was omitted for value and reference returns).
- The `ReturnSlotPattern` always has a subpattern with the same form.
`OutParamPattern` already plays that role for initializing forms, and
`TuplePattern` will play that role for tuple forms. This change
introduces `ValueReturnPattern` and `RefReturnPattern` to represent
value and reference return forms.
- As before, the `ReturnSlotPattern` has a corresponding `ReturnSlot`
that represents the output that is initialized by a `return` statement.
Its structure parallels the structure of the `ReturnSlotPattern`, so we
need `ValueReturn` and `RefReturn` insts that correspond to
`ValueReturnPattern` and `RefReturnPattern`.

This is a step toward supporting generic return forms, where the
`ReturnSlotPattern`'s subpattern may be an action: this change ensures
that evaluating the action for a specific form produces the same SemIR
as if the form were concrete to begin with. More speculatively, this
should simplify the implementation of `return` statements with compound
return forms.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/check/handle_impl.cpp

@@ -158,7 +158,7 @@ static auto PopImplIntroducerAndParamsAsNameComponent(
     // because `impl`s are never actually called at runtime.
     auto match_results =
         CalleePatternMatch(context, *implicit_param_patterns_id,
-                           SemIR::InstBlockId::None, SemIR::InstBlockId::None);
+                           SemIR::InstBlockId::None, SemIR::InstId::None);
     CARBON_CHECK(match_results.call_params_id == SemIR::InstBlockId::Empty);
     CARBON_CHECK(match_results.call_param_patterns_id ==
                  SemIR::InstBlockId::Empty);