OpenMP: Look up 'declare mapper' definitions at resolution time not parse time

This patch moves 'declare mapper' lookup for OpenMP clauses from parse
time to resolution time for Fortran, and adds diagnostics for missing
named mappers.  This changes clause lookup in a particular case -- where
several 'declare mapper's are defined in a context, mappers declared
earlier may now instantiate mappers declared later, whereas previously
they would not.  I think the new behaviour makes more sense -- at an
invocation site, all mappers are visible no matter the declaration order
in some particular block.  I've adjusted tests to account for this.

I think the new arrangement better matches the Fortran FE's usual way of
doing things -- mapper lookup is a semantic concept, not a syntactical
one, so shouldn't be handled in the syntax-handling code.

The patch also fixes a case where the user explicitly writes 'default'
as the name on the mapper modifier for a clause.

2023-08-10  Julian Brown  <julian@codesourcery.com>

gcc/fortran/
	* gfortran.h (gfc_omp_namelist_udm): Add MAPPER_ID field to store the
	mapper name to use for lookup during resolution.
	* match.cc (gfc_free_omp_namelist): Handle OMP_LIST_TO and
	OMP_LIST_FROM when freeing mapper references.
	* module.cc (load_omp_udms, write_omp_udm): Handle MAPPER_ID field.
	* openmp.cc (gfc_match_omp_clauses): Handle explicitly-specified
	'default' name.  Don't do mapper lookup here, but record mapper name if
	the user specifies one.
	(resolve_omp_clauses): Do mapper lookup here instead.  Report error for
	missing named mapper.

gcc/testsuite/
	* gfortran.dg/gomp/declare-mapper-31.f90: New test.

libgomp/
	* testsuite/libgomp.fortran/declare-mapper-30.f90: New test.
	* testsuite/libgomp.fortran/declare-mapper-4.f90: Adjust test for new
	lookup behaviour.

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