[PATCH] libcpp: suppress builtin macro redefined warnings for __LINE__

From 0821df518b264e754d698d399f98be1a62945e32 Mon Sep 17 00:00:00 2001
From: Longjun Luo <luolongjuna@gmail.com>
Date: Thu, 12 Jan 2023 23:59:54 +0800
Subject: [PATCH] libcpp: suppress builtin macro redefined warnings for
 __LINE__

As implied in
gcc.gnu.org/legacy-ml/gcc-patches/2008-09/msg00076.html,
gcc provides -Wno-builtin-macro-redefined to suppress warning when
redefining builtin macro. However, at that time, there was no
scenario for __LINE__ macro.

But, when we try to build a live-patch, we compare sections by using
-ffunction-sections. Some same functions are considered changed because
of __LINE__ macro.

At present, to detect such a changed caused by __LINE__ macro, we
have to analyse code and maintain a function list. For example,
in kpatch, check this commit
github.com/dynup/kpatch/commit/0e1b95edeafa36edb7bcf11da6d1c00f76d7e03d.

So, in this scenario, when we try to compared sections, it would
be better to support suppress builtin macro redefined warnings for
__LINE__ macro.

libcpp:
	* init.cc (builtin_array): Do not always warn for a redefinition
	of __LINE__.

gcc/testsuite

	* gcc.dg/builtin-redefine.c: Test for redefintion warnings
	for __LINE__.
	* gcc.dg/builtin-redefine-1.c: New test.