Remove outdated FIXME; should have removed that in r160782

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168698 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGenCXX/mangle-ms-back-references-pr13207.cpp

 // RUN: %clang_cc1 -emit-llvm %s -o - -cxx-abi microsoft -triple=i386-pc-win32 | FileCheck %s
 
-// FIXME: add tests for return types with complex templates when PR13389 is fixed.
-
 template<class X, class Y, class Z>
 class A {};
 template<class X>