cgraphunit: Avoid code generation differences based on -w/TREE_NO_WARNING [PR94277]

cgraphunit: Avoid code generation differences based on -w/TREE_NO_WARNING [PR94277]

The following testcase FAILs with -fcompare-debug, but not because -g vs.
-g0 would make a difference, but because the second compilation is done with
-w in order not to emit warnings twice and -w seems to affect the *.gkd dump
content.
This is because TREE_NO_WARNING flag, or warn_unused_function does affect
not just whether a warning/pedwarn is printed, but also whether we set
TREE_PUBLIC on such decls.
The following patch makes sure we set it regardless of anything warning
related (TREE_NO_WARNING or warn_unused_function).

2020-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR debug/94277
	* cgraphunit.c (check_global_declaration): For DECL_EXTERNAL and
	non-TREE_PUBLIC non-DECL_ARTIFICIAL FUNCTION_DECLs, set TREE_PUBLIC
	regardless of whether TREE_NO_WARNING is set on it or whether
	warn_unused_function is true or not.

	* gcc.dg/pr94277.c: New test.