[cxx-interop] Fix tests that passed -module-name Swift

`Swift` is the name of the Swift standard library module. Creating another module with this name breaks assumptions in the compiler and caused these two tests to fail.

Author: egorzhdan

test/Interop/Cxx/class/Inputs/module.modulemap

@@ -5,12 +5,18 @@ module AccessSpecifiers {
 
 module TypeClassification {
   header "type-classification.h"
-  requires cplusplus
+  // No 'requires cplusplus', because we don't want an implicit 'import Cxx'
+  // when this module gets imported into Swift – the tests that use this module
+  // override the 'Swift' module which causes a circular dependency between
+  // 'Swift' and 'Cxx'.
 }
 
 module Constructors {
   header "constructors.h"
-  requires cplusplus
+  // No 'requires cplusplus', because we don't want an implicit 'import Cxx'
+  // when this module gets imported into Swift – the tests that use this module
+  // override the 'Swift' module which causes a circular dependency between
+  // 'Swift' and 'Cxx'.
 }
 
 module ConstructorsObjC {