Refactor ReturnTypeInfo and InitRepr. (#4169)

Rename `ReturnInfo` to `ReturnTypeInfo`. Move it and `InitRepr` into
`type_info.h` alongside `ValueRepr`. Replace `ReturnSlot` with
`InitRepr`, and extend `InitRepr` to be able to represent the
incomplete-type case instead of CHECK-failing. Remove `has_return_slot`
from `InitRepr` and instead only provide that as part of
`ReturnTypeInfo`.

toolchain/check/function.cpp

@@ -73,12 +73,13 @@ auto CheckFunctionTypeMatches(Context& context,
 auto CheckFunctionReturnType(Context& context, SemIRLoc loc,
                              SemIR::Function& function,
                              SemIR::SpecificId specific_id)
-    -> SemIR::ReturnInfo {
-  auto return_info = function.GetReturnInfo(context.sem_ir(), specific_id);
+    -> SemIR::ReturnTypeInfo {
+  auto return_info = SemIR::ReturnTypeInfo::ForFunction(context.sem_ir(),
+                                                        function, specific_id);
 
   // If we couldn't determine the return information due to the return type
   // being incomplete, try to complete it now.
-  if (return_info.return_slot == SemIR::ReturnSlot::Incomplete) {
+  if (return_info.init_repr.kind == SemIR::InitRepr::Incomplete) {
     auto diagnose_incomplete_return_type = [&] {
       CARBON_DIAGNOSTIC(IncompleteTypeInFunctionReturnType, Error,
                         "Function returns incomplete type `{0}`.",
@@ -91,7 +92,8 @@ auto CheckFunctionReturnType(Context& context, SemIRLoc loc,
     // definition or call to this function.
     if (context.TryToCompleteType(return_info.type_id,
                                   diagnose_incomplete_return_type)) {
-      return_info = function.GetReturnInfo(context.sem_ir(), specific_id);
+      return_info = SemIR::ReturnTypeInfo::ForFunction(context.sem_ir(),
+                                                       function, specific_id);
     }
   }