Move tests from CXXInterop to Interop/Cxx.

This is the canonical location that we've agreed on.

Author: martinboehme

test/CXXInterop/Inputs/module.modulemap

@@ -6,6 +6,10 @@ module TypeClassification {
   header "type-classification.h"
 }
 
+module CxxConstructors {
+  header "cxx-constructors.h"
+}
+
 module MemberwiseInitializer {
   header "memberwise-initializer.h"
 }