One failure with a -std=gnu23 default that indicates a
quality-of-implementation regression in C23 mode is gcc.dg/pr39084.c,
which loses the expected "originally defined here" message on struct
redefinition errors (which occur in a different place in the front end
for C23 because it is necessary to see the members of the struct to
determine whether the redefinition is valid).  That message seems a
good thing to have both in and out of C23 mode, so add logic to
restore it in the C23 case.

Bootstrapped with no regressions for x86-64-pc-linux-gnu.

gcc/c/
	* c-decl.cc (c_struct_parse_info): Add member refloc.
	(start_struct): Store refloc in struct_parse_info.
	(finish_struct): Give "originally defined" message for C23 struct
	redefinition errors.

gcc/testsuite/
	* gcc.dg/gnu17-tag-1.c, gcc.dg/gnu23-tag-5.c: New tests.