Add support for allocate clause (OpenMP 5.0).

Add support for allocate clause (OpenMP 5.0).

This patch adds support for OpenMP 5.0 allocate clause for fortran. It does not
yet support the allocator-modifier as specified in OpenMP 5.1. The allocate
clause is already supported in C/C++.

gcc/fortran/ChangeLog:

	* dump-parse-tree.c (show_omp_clauses): Handle OMP_LIST_ALLOCATE.
	* gfortran.h (OMP_LIST_ALLOCATE): New enum value.
	* openmp.c (enum omp_mask1): Add OMP_CLAUSE_ALLOCATE.
	(gfc_match_omp_clauses): Handle OMP_CLAUSE_ALLOCATE
	(OMP_PARALLEL_CLAUSES, OMP_DO_CLAUSES, OMP_SECTIONS_CLAUSES)
	(OMP_TASK_CLAUSES, OMP_TASKLOOP_CLAUSES, OMP_TARGET_CLAUSES)
	(OMP_TEAMS_CLAUSES, OMP_DISTRIBUTE_CLAUSES)
	(OMP_SINGLE_CLAUSES): Add OMP_CLAUSE_ALLOCATE.
	(OMP_TASKGROUP_CLAUSES): New.
	(gfc_match_omp_taskgroup): Use OMP_TASKGROUP_CLAUSES instead of
	OMP_CLAUSE_TASK_REDUCTION.
	(resolve_omp_clauses): Handle OMP_LIST_ALLOCATE.
	(resolve_omp_do): Avoid warning when loop iteration variable is
	in allocate clause.
	* trans-openmp.c (gfc_trans_omp_clauses): Handle translation of
	allocate clause.
	(gfc_split_omp_clauses): Update for OMP_LIST_ALLOCATE.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/allocate-1.f90: New test.
	* gfortran.dg/gomp/allocate-2.f90: New test.
	* gfortran.dg/gomp/allocate-3.f90: New test.
	* gfortran.dg/gomp/collapse1.f90: Update error message.
	* gfortran.dg/gomp/openmp-simd-4.f90: Likewise.
	* gfortran.dg/gomp/clauses-1.f90: Uncomment allocate clause.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/allocate-1.c: New test.
	* testsuite/libgomp.fortran/allocate-1.f90: New test.
	* libgomp.texi: Remove string that says that allocate clause
	support is for C/C++ only.