Decouple PerformCallToFunction from ReturnTypeInfo (#6572)

`ReturnTypeInfo` is built around the assumption that a function call
results in exactly one initializing expression, but with `ref` returns
there may be zero, and in the future composite return forms will enable
there to be more than one. This change removes some usages of
`ReturnTypeInfo`, and restructures the calling code to be prepared for
multiple initializing returns.

toolchain/check/pattern_match.cpp

@@ -629,17 +629,22 @@ auto CallerPatternMatch(Context& context, SemIR::SpecificId specific_id,
                         SemIR::InstBlockId return_patterns_id,
                         SemIR::InstId self_arg_id,
                         llvm::ArrayRef<SemIR::InstId> arg_refs,
-                        SemIR::InstId return_slot_arg_id,
+                        llvm::ArrayRef<SemIR::InstId> return_arg_ids,
                         bool is_operator_syntax) -> SemIR::InstBlockId {
   MatchContext match(MatchKind::Caller, specific_id);
 
   auto return_patterns = context.inst_blocks().GetOrEmpty(return_patterns_id);
   // Track the return storage, if present.
-  if (return_slot_arg_id.has_value()) {
-    CARBON_CHECK(return_patterns.size() == 1,
-                 "TODO: implement support for multiple return patterns");
-    match.AddWork(
-        {.pattern_id = return_patterns[0], .scrutinee_id = return_slot_arg_id});
+  for (auto [return_pattern_id, return_arg_id] :
+       llvm::zip_equal(return_patterns, return_arg_ids)) {
+    if (return_arg_id.has_value()) {
+      match.AddWork(
+          {.pattern_id = return_pattern_id, .scrutinee_id = return_arg_id});
+    } else {
+      CARBON_CHECK(return_arg_ids.size() == 1,
+                   "TODO: do the match even if return_arg_id is None, so that "
+                   "subsequent args are at the right index in the arg block");
+    }
   }
 
   // Check type conversions per-element.