Support C++ calling Carbon functions with ref parameters (#7107)

When creating the C++ thunk, make the parameters references if the
corresponding callee parameters are `ref`s.

When creating the Carbon thunk, tag the call arguments as `ref` if the
corresponding callee parameters are `ref`s.

toolchain/check/testdata/interop/cpp/class/export/method.carbon

@@ -0,0 +1,84 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/class/export/method.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/class/export/method.carbon
+
+// --- static_method.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class C {
+  fn M();
+}
+
+inline Cpp '''
+void F() {
+  Carbon::C::M();
+}
+''';
+
+// --- method_lvalue.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class C {
+  fn M[self: Self]();
+}
+
+inline Cpp '''
+void F() {
+  Carbon::C c;
+  c.M();
+}
+''';
+
+// --- method_rvalue.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class C {
+  fn M[self: Self]();
+}
+
+inline Cpp '''
+void F() {
+  Carbon::C().M();
+}
+''';
+
+// --- ref_method_lvalue.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class C {
+  fn M[ref self: Self]();
+}
+
+inline Cpp '''
+void F() {
+  Carbon::C c;
+  c.M();
+}
+''';
+
+// --- todo_ref_method_rvalue.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class C {
+  fn M[ref self: Self]();
+}
+
+inline Cpp '''
+void F() {
+  // TODO: this should be disallowed.
+  Carbon::C().M();
+}
+''';

toolchain/check/testdata/interop/cpp/function/export/function.carbon

@@ -20,10 +20,6 @@ fn F3(_: i32) {}
 fn HasGenericArg(T:! type, a: T) { a; }
 fn HasDeducedArg[T:! type](a: T) { a; }
 
-class C {
-  fn Method[self: Self]() { self; }
-}
-
 // --- function.carbon
 
 library "[[@TEST_NAME]]";
@@ -57,6 +53,21 @@ void G() {
 }
 ''';
 
+// --- ref_arg.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+fn F(ref n: i32);
+
+inline Cpp '''
+void G() {
+  int n = 0;
+  Carbon::F(n);
+}
+''';
+
 // --- using.carbon
 
 library "[[@TEST_NAME]]";
@@ -113,14 +124,3 @@ void G() {
   Carbon::Other::HasDeducedArg(123);
 }
 ''';
-
-// --- method.carbon
-
-library "[[@TEST_NAME]]";
-
-import Other;
-import Cpp inline '''
-void G() {
-  Carbon::Other::C().Method();
-}
-''';