Refactor function return type representation (#6463)

This separates the return type from the return pattern, and replaces the
return pattern with a block of return patterns. This is a step toward
support for `ref` returns (where there's no corresponding return
pattern) and compund-form returns (where there may be multiple return
patterns).

toolchain/check/handle_impl.cpp

@@ -156,7 +156,7 @@ static auto PopImplIntroducerAndParamsAsNameComponent(
     // because `impl`s are never actually called at runtime.
     auto call_params_id =
         CalleePatternMatch(context, *implicit_param_patterns_id,
-                           SemIR::InstBlockId::None, SemIR::InstId::None);
+                           SemIR::InstBlockId::None, SemIR::InstBlockId::None);
     CARBON_CHECK(call_params_id == SemIR::InstBlockId::Empty ||
                  llvm::all_of(context.inst_blocks().Get(call_params_id),
                               [](SemIR::InstId inst_id) {
@@ -186,7 +186,6 @@ static auto PopImplIntroducerAndParamsAsNameComponent(
           .params_loc_id = Parse::NodeId::None,
           .param_patterns_id = SemIR::InstBlockId::None,
           .call_params_id = SemIR::InstBlockId::None,
-          .return_slot_pattern_id = SemIR::InstId::None,
           .pattern_block_id = pattern_block_id};
 }