[AST] For USR generation, ignore symbols coming from the builtin module.

Swift SVN r17858

include/swift/AST/Module.h

@@ -394,6 +394,9 @@ public:
   /// \returns true if this module is the "swift" standard library module.
   bool isStdlibModule() const;
 
+  /// \returns true if this module is the "builtin" module.
+  bool isBuiltinModule() const;
+
   /// \returns true if this module is a system module; note that the StdLib is
   /// considered a system module.
   bool isSystemModule() const;