Handle signature mismatch when a Carbon function overrides a C++ virtual function. (#7499)

When a Carbon virtual function overrides a C++ virtual function, we need
to export it with the C++ signature in order for it to work as an
override. Instead of mapping the C++ signature into Carbon and then back
again, use the original C++ signature from the base class as the
signature exported to C++.

Also add documentation explaining how we use thunks in C++ interop,
including in this new virtual function handling logic.

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

@@ -0,0 +1,32 @@
+// 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/destroy.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/override.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/class/export/override.carbon
+
+// --- const_ref_param.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+inline Cpp '''
+struct X {};
+
+struct A {
+  virtual void f(const X& x) = 0;
+};
+''';
+
+class B {
+  extend base: Cpp.A;
+  override fn f(unused self, unused x: Cpp.X) {}
+}
+
+inline Cpp '''
+Carbon::B b;
+''';