C++ interop: Add return reference types support (#6178)

This is a follow up of #6082, which added support for reference types,
but not for return types.

C++ Interop Demo:

```carbon
// main.carbon

library "Main";

import Core library "io";

import Cpp inline '''
struct C {
  auto Inc() -> void { ++x; }
  int x = 0;
};
auto GetC() -> C& {
  static C c;
  return c;
}
''';

fn Run() -> i32 {
  Core.Print(Cpp.GetC()->x);
  Cpp.GetC()->Inc();
  Core.Print(Cpp.GetC()->x);
  Cpp.GetC()->Inc();
  Core.Print(Cpp.GetC()->x);
  return 0;
}
```

```shell
$ bazel build toolchain:carbon && bazel-bin/toolchain/carbon compile main.carbon && bazel-bin/toolchain/carbon link main.o --output=demo && ./demo
0
1
2
```

**Without this change**:
```shell
main.carbon:19:14: error: semantics TODO: `Unsupported: return type: C &`
  Core.Print(Cpp.GetC()->x);
             ^~~~~~~~~~
```

Part of #6148.

toolchain/check/cpp/import.cpp

@@ -1446,16 +1446,26 @@ static auto MakeParamPatternsBlockId(Context& context, SemIR::LocId loc_id,
 // TODO: Support more return types.
 static auto GetReturnTypeExpr(Context& context, SemIR::LocId loc_id,
                               clang::FunctionDecl* clang_decl) -> TypeExpr {
-  clang::QualType ret_type = clang_decl->getReturnType();
-  if (!ret_type->isVoidType()) {
-    TypeExpr mapped_type = MapType(context, loc_id, ret_type);
-    if (!mapped_type.inst_id.has_value()) {
+  clang::QualType orig_ret_type = clang_decl->getReturnType();
+  if (!orig_ret_type->isVoidType()) {
+    // We map `T&` return type to `addr param: T*`, and `T&&` parameters to
+    // `param: T`.
+    // TODO: Revisit this and decide what we really want to do here.
+    clang::QualType ret_type = orig_ret_type.getNonReferenceType();
+
+    auto [orig_type_inst_id, type_id] = MapType(context, loc_id, ret_type);
+    if (!orig_type_inst_id.has_value()) {
       context.TODO(loc_id, llvm::formatv("Unsupported: return type: {0}",
-                                         ret_type.getAsString()));
+                                         orig_ret_type.getAsString()));
       return {.inst_id = SemIR::ErrorInst::TypeInstId,
               .type_id = SemIR::ErrorInst::TypeId};
     }
-    return mapped_type;
+
+    if (orig_ret_type->isLValueReferenceType()) {
+      type_id = GetPointerType(context, orig_type_inst_id);
+    }
+
+    return {orig_type_inst_id, type_id};
   }
 
   auto* ctor = dyn_cast<clang::CXXConstructorDecl>(clang_decl);

toolchain/check/cpp/thunk.cpp

@@ -241,6 +241,14 @@ static auto GetNonnullType(clang::ASTContext& ast_context,
                                        pointer_type, pointer_type);
 }
 
+// Given a type, returns the corresponding _Nonnull-qualified pointer type,
+// ignoring references.
+static auto GetNonNullablePointerType(clang::ASTContext& ast_context,
+                                      clang::QualType type) {
+  return GetNonnullType(ast_context,
+                        ast_context.getPointerType(type.getNonReferenceType()));
+}
+
 // Given the type of a callee parameter, returns the type to use for the
 // corresponding thunk parameter.
 static auto GetThunkParameterType(clang::ASTContext& ast_context,
@@ -249,8 +257,7 @@ static auto GetThunkParameterType(clang::ASTContext& ast_context,
   if (IsSimpleAbiType(ast_context, callee_type)) {
     return callee_type;
   }
-  return GetNonnullType(ast_context, ast_context.getPointerType(
-                                         callee_type.getNonReferenceType()));
+  return GetNonNullablePointerType(ast_context, callee_type);
 }
 
 // Creates the thunk parameter types given the callee function.
@@ -272,9 +279,8 @@ static auto BuildThunkParameterTypes(clang::ASTContext& ast_context,
   }
 
   if (!callee_info.has_simple_return_type) {
-    thunk_param_types.push_back(GetNonnullType(
-        ast_context,
-        ast_context.getPointerType(callee_info.effective_return_type)));
+    thunk_param_types.push_back(GetNonNullablePointerType(
+        ast_context, callee_info.effective_return_type));
   }
 
   CARBON_CHECK(thunk_param_types.size() == callee_info.num_thunk_params());
@@ -524,7 +530,7 @@ static auto BuildThunkBody(clang::Sema& sema,
 
   auto* return_object_addr = BuildThunkParamRef(
       sema, thunk_function_decl, callee_info.GetThunkReturnParamIndex());
-  auto return_type = callee_info.effective_return_type;
+  auto return_type = callee_info.effective_return_type.getNonReferenceType();
   auto* return_type_info =
       sema.Context.getTrivialTypeSourceInfo(return_type, clang_loc);
   auto placement_new = sema.BuildCXXNew(