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/call.cpp

@@ -115,7 +115,7 @@ auto PerformCall(Context& context, Parse::NodeId node_id,
 
   // If there is a return slot, build storage for the result.
   SemIR::InstId return_storage_id = SemIR::InstId::Invalid;
-  SemIR::ReturnInfo return_info = [&] {
+  SemIR::ReturnTypeInfo return_info = [&] {
     DiagnosticAnnotationScope annotate_diagnostics(
         &context.emitter(), [&](auto& builder) {
           CARBON_DIAGNOSTIC(IncompleteReturnTypeHere, Note,
@@ -124,21 +124,23 @@ auto PerformCall(Context& context, Parse::NodeId node_id,
         });
     return CheckFunctionReturnType(context, callee_id, callable, specific_id);
   }();
-  switch (return_info.return_slot) {
-    case SemIR::ReturnSlot::Present:
+  switch (return_info.init_repr.kind) {
+    case SemIR::InitRepr::InPlace:
       // Tentatively put storage for a temporary in the function's return slot.
       // This will be replaced if necessary when we perform initialization.
       return_storage_id = context.AddInst<SemIR::TemporaryStorage>(
           node_id, {.type_id = return_info.type_id});
       break;
-    case SemIR::ReturnSlot::Absent:
+    case SemIR::InitRepr::None:
       // For functions with an implicit return type, the return type is the
       // empty tuple type.
       if (!return_info.type_id.is_valid()) {
         return_info.type_id = context.GetTupleType({});
       }
       break;
-    case SemIR::ReturnSlot::Incomplete:
+    case SemIR::InitRepr::ByCopy:
+      break;
+    case SemIR::InitRepr::Incomplete:
       // Don't form an initializing expression with an incomplete type.
       // CheckFunctionReturnType will have diagnosed this for us if needed.
       return_info.type_id = SemIR::TypeId::Error;