[libc++] Adds __format_string as nasty macro.

[Mordante]

Summary

Both D121530 and D125606 had issues with this macro.

libcxx/test/libcxx/nasty_macros.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // Test that headers are not tripped up by the surrounding code defining various

 
 // Test that libc++ doesn't use names that collide with Win32 API macros.
 // Obviously we can only define these on non-Windows platforms.
 #ifndef _WIN32
 # define __allocator NASTY_MACRO
 # define __bound NASTY_MACRO
 # define __deallocate NASTY_MACRO
 # define __deref NASTY_MACRO
+# define __format_string NASTY_MACRO
 # define __full NASTY_MACRO
 # define __in NASTY_MACRO
 # define __inout NASTY_MACRO
 # define __nz NASTY_MACRO
 # define __out NASTY_MACRO
 # define __part NASTY_MACRO
 # define __post NASTY_MACRO
 # define __pre NASTY_MACRO