Add a PR reference for XFAILed test

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209730 91177308-0d34-0410-b5e6-96231b3b80d8

test/Parser/cxx-ambig-decl-expr-xfail.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify %s
 // XFAIL: *
+// FIXME: This is PR7655
+
 struct X { 
   template<typename T> X(T);
   X(int, int);