Export methods taking self as const member functions (#7577)

A method declared with `self` does not modify the object, but it was
exported to C++ as a non-const member function, so calling it on a const
reference would fail.

```carbon
class C {
  fn Get(self);
}

inline Cpp '''
void F(const Carbon::C& c) {
  c.Get();
}
''';
```
```
error: 'this' argument to member function 'Get' has type 'const Carbon::C', but function is not marked const
```

Import already maps `f() const` to `fn f(self)`, and this PR implements
the same behavior for exporting. No ref-qualifier is added, since that
maps to `ref self`, so that is unchanged.

`GetThisArg()` now builds `this` from the method instead of the parent
record, so that it picks up the method's const-qualifier.

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

@@ -53,6 +53,20 @@ void F() {
 }
 ''';
 
+// --- method_const_lvalue.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class C {
+  fn M(self);
+}
+
+inline Cpp '''
+void F(const Carbon::C& c) {
+  c.M();
+}
+''';
+
 // --- ref_method_lvalue.carbon
 library "[[@TEST_NAME]]";
 import Cpp;
@@ -89,6 +103,27 @@ void F() {
 }
 ''';
 
+// --- fail_ref_method_const_lvalue.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class C {
+  fn M(ref self);
+}
+
+inline Cpp '''
+void F(const Carbon::C& c) {
+  // CHECK:STDERR: fail_ref_method_const_lvalue.carbon:[[@LINE+7]]:3: error: 'this' argument to member function 'M' has type 'const Carbon::C', but function is not marked const [CppInteropParseError]
+  // CHECK:STDERR:    17 |   c.M();
+  // CHECK:STDERR:       |   ^
+  // CHECK:STDERR: fail_ref_method_const_lvalue.carbon:[[@LINE-8]]:17: note: 'M' declared here [CppInteropParseNote]
+  // CHECK:STDERR:     5 |   fn M(ref self);
+  // CHECK:STDERR:       |                 ^
+  // CHECK:STDERR:
+  c.M();
+}
+''';
+
 // --- method_alias.carbon
 library "[[@TEST_NAME]]";
 import Cpp;