OpenMP: give error when variant is the same as the base function [PR118839]

OpenMP: give error when variant is the same as the base function [PR118839]

As noted in the issue, the C++ front end has deeper problems: it's
supposed to do the name lookup of the variant at the call site but is
instead doing it when parsing the "declare variant" construct, before
registering the decl for the base function.  The C++ part of the
patch is a band-aid to catch the case where there is a previous declaration
of the function and it doesn't give an undefined symbol error instead.
Some real solution ought to be included as part of fixing PR118791.

gcc/c/
	PR middle-end/118839
	* c-parser.cc (c_finish_omp_declare_variant): Error if variant
	is the same as base.

gcc/cp/
	PR middle-end/118839
	* decl.cc (omp_declare_variant_finalize_one): Error if variant
	is the same as base.

gcc/fortran/
	PR middle-end/118839
	* trans-openmp.cc (gfc_trans_omp_declare_variant): Error if variant
	is the same as base.

gcc/testsuite/
	PR middle-end/118839
	* gcc.dg/gomp/declare-variant-3.c: New.
	* gfortran.dg/gomp/declare-variant-22.f90: New.