Exclude private decls from interface token hash.

After parsing a private decl, reset the token hash state to what it was
before the decl was parsed. This way, adding a private decl or editing
its type or name will not trigger a rebuild of downstream files.

<rdar://problem/22239821> Modifying private APIs shouldn't cause dependents to recompile

Swift SVN r31358

include/swift/AST/Module.h

@@ -1083,6 +1083,9 @@ public:
     InterfaceHash.update(a);
   }
 
+  const llvm::MD5 &getInterfaceHashState() { return InterfaceHash; }
+  void setInterfaceHashState(const llvm::MD5 &state) { InterfaceHash = state; }
+
   void getInterfaceHash(llvm::SmallString<32> &str) {
     llvm::MD5::MD5Result result;
     InterfaceHash.final(result);