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();
+}
+''';