Fortran: Store OpenMP's 'declare variant' in module file [PR115271]

Fortran: Store OpenMP's 'declare variant' in module file [PR115271]

Write the 'omp declare variant' data into the .mod file: Base function,
variant function(s), supporting the clauses match, append_args, and
adjust_args.

	PR fortran/115271

gcc/fortran/ChangeLog:

	* module.cc (mio_omp_declare_simd_clauses): New, moved from ...
	(mio_omp_declare_simd): ... here. Update call, write empty '( )'
	if there is no declare simd but a declare variant.
	(mio_omp_declare_variant): New.
	(mio_symbol): Call it.
	* openmp.cc (gfc_match_omp_context_selector): Add comment about
	module.cc to TODO note.
	* trans-stmt.h (gfc_trans_omp_declare_variant): Take additional
	parent_ns argument.
	* trans-decl.cc (create_function_arglist,
	gfc_create_function_decl): Update call.
	* trans-openmp.cc (gfc_trans_omp_declare_variant): Take new
	argument, add some special case handling for attr.use_assoc.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/declare-variant-mod-1-use.f90: New test.
	* gfortran.dg/gomp/declare-variant-mod-1.f90: New test.
	* gfortran.dg/gomp/declare-variant-mod-2-use.f90: New test.
	* gfortran.dg/gomp/declare-variant-mod-2.f90: New test.