Add support for C++ calling Carbon functions with parameters (#7024)

This works by generating two thunks, one in C++ and one in Carbon. For
example, given this input:
```c++
// Carbon:
fn Callme(f: f32) {}

// C++:
void F() {
  // This will call `Callme__cpp_thunk`
  Carbon::Callme(1.0);
}
```

These functions are generated:
```c++
// Carbon:
fn Callme__carbon_thunk(ref f: f32) {
  // Call the target function.
  Callme(f);
}

// C++:

// C++ declaration for the Carbon thunk.
void Callme__carbon_thunk(float& f);

void Callme__cpp_thunk(float f) {
  // Call the Carbon thunk with args passed by reference.
  Callme__carbon_thunk(f);
}
```

For now, all arguments are passed by reference, even if they are simple
types like pointers or i32.

Functions with non-void return types are not supported yet.

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

@@ -17,6 +17,13 @@ fn F1() {}
 fn F2() -> i32 { return 0; }
 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]]";
@@ -48,22 +55,76 @@ void G() {
 }
 ''';
 
-// --- fail_todo_args.carbon
+// --- args.carbon
 
 library "[[@TEST_NAME]]";
 
-// CHECK:STDERR: fail_todo_args.carbon:[[@LINE+5]]:1: in import [InImport]
-// CHECK:STDERR: other.carbon:5:1: error: semantics TODO: `unsupported: C++ calling a Carbon function with parameters` [SemanticsTodo]
-// CHECK:STDERR: fn F3(_: i32) {}
-// CHECK:STDERR: ^~~~~~~~~~~~~~~
+import Other;
+import Cpp inline '''
+void G() {
+  Carbon::Other::F3(123);
+}
+''';
+
+// --- fail_todo_generic.carbon
+
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: fail_todo_generic.carbon:[[@LINE+5]]:1: in import [InImport]
+// CHECK:STDERR: other.carbon:7:1: error: semantics TODO: `unsupported: C++ calling a Carbon function with generic parameters` [SemanticsTodo]
+// CHECK:STDERR: fn HasGenericArg(T:! type, a: T) { a; }
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // CHECK:STDERR:
 import Other;
 import Cpp inline '''
 void G() {
-  // CHECK:STDERR: fail_todo_args.carbon:[[@LINE+4]]:18: error: no member named 'F3' in namespace 'Carbon::Other' [CppInteropParseError]
-  // CHECK:STDERR:    16 |   Carbon::Other::F3();
-  // CHECK:STDERR:       |                  ^~
+  // CHECK:STDERR: fail_todo_generic.carbon:[[@LINE+8]]:18: error: no member named 'HasGenericArg' in namespace 'Carbon::Other' [CppInteropParseError]
+  // CHECK:STDERR:    20 |   Carbon::Other::HasGenericArg<int>(123);
+  // CHECK:STDERR:       |                  ^~~~~~~~~~~~~
   // CHECK:STDERR:
-  Carbon::Other::F3();
+  // CHECK:STDERR: fail_todo_generic.carbon:[[@LINE+4]]:35: error: expected '(' for function-style cast or type construction [CppInteropParseError]
+  // CHECK:STDERR:    20 |   Carbon::Other::HasGenericArg<int>(123);
+  // CHECK:STDERR:       |                                ~~~^
+  // CHECK:STDERR:
+  Carbon::Other::HasGenericArg<int>(123);
+}
+''';
+
+// --- fail_todo_deduced.carbon
+
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: fail_todo_deduced.carbon:[[@LINE+5]]:1: in import [InImport]
+// CHECK:STDERR: other.carbon:8:1: error: semantics TODO: `unsupported: C++ calling a Carbon function with generic parameters` [SemanticsTodo]
+// CHECK:STDERR: fn HasDeducedArg[T:! type](a: T) { a; }
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+import Other;
+import Cpp inline '''
+void G() {
+  // CHECK:STDERR: fail_todo_deduced.carbon:[[@LINE+4]]:18: error: no member named 'HasDeducedArg' in namespace 'Carbon::Other' [CppInteropParseError]
+  // CHECK:STDERR:    16 |   Carbon::Other::HasDeducedArg(123);
+  // CHECK:STDERR:       |                  ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  Carbon::Other::HasDeducedArg(123);
+}
+''';
+
+// --- fail_todo_method.carbon
+
+library "[[@TEST_NAME]]";
+
+import Other;
+import Cpp inline '''
+void G() {
+  // CHECK:STDERR: fail_todo_method.carbon:[[@LINE+8]]:3: error: invalid use of incomplete type 'Carbon::Other::C' [CppInteropParseError]
+  // CHECK:STDERR:    15 |   Carbon::Other::C().Method();
+  // CHECK:STDERR:       |   ^~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  // CHECK:STDERR: fail_todo_method.carbon:[[@LINE+4]]:21: error: member access into incomplete type 'Carbon::Other::C' [CppInteropParseError]
+  // CHECK:STDERR:    15 |   Carbon::Other::C().Method();
+  // CHECK:STDERR:       |                     ^
+  // CHECK:STDERR:
+  Carbon::Other::C().Method();
 }
 ''';