OpenMP: Cleanups related to the 'present' modifier

Reduce number of enum values passed to libgomp as
GOMP_MAP_PRESENT_{TO,TOFROM,FROM,ALLOC} have the same semantic as
GOMP_MAP_FORCE_PRESENT (i.e. abort if not present, otherwise ignore);
that's different to GOMP_MAP_ALWAYS_PRESENT_{TO,TOFROM,FROM} which also
abort if not present but copy data when present. This is is a follow-up to
the commit r14-1579-g4ede915d5dde93 done 6 days ago.

Additionally, the commit improves a libgomp run-time and a C/C++ compile-time
error wording and extends testcases a tiny bit.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_omp_clause_map): Reword error message for
	clearness especially with 'omp target (enter/exit) data.'

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_omp_clause_map): Reword error message for
	clearness especially with 'omp target (enter/exit) data.'
	* semantics.cc (handle_omp_array_sections): Handle
	GOMP_MAP_{ALWAYS_,}PRESENT_{TO,TOFROM,FROM,ALLOC} enum values.

gcc/ChangeLog:

	* gimplify.cc (gimplify_adjust_omp_clauses_1): Use
	GOMP_MAP_FORCE_PRESENT for 'present alloc' implicit mapping.
	(gimplify_adjust_omp_clauses): Change
	GOMP_MAP_PRESENT_{TO,TOFROM,FROM,ALLOC} to the equivalent
	GOMP_MAP_FORCE_PRESENT.
	* omp-low.cc (lower_omp_target): Remove handling of no-longer valid
	GOMP_MAP_PRESENT_{TO,TOFROM,FROM,ALLOC}; update map kinds used for
	to/from clauses with present modifier.

include/ChangeLog:

	* gomp-constants.h (enum gomp_map_kind): Change the enum values
	GOMP_MAP_PRESENT_{TO,TOFROM,FROM,ALLOC} to be compiler only.
	(GOMP_MAP_PRESENT_P): Update to include also GOMP_MAP_FORCE_PRESENT.

libgomp/ChangeLog:

	* target.c (gomp_to_device_kind_p, gomp_map_vars_internal): Replace
	GOMP_MAP_PRESENT_{FROM,TO,TOFROM,ACLLOC} by GOMP_MAP_FORCE_PRESENT.
	(gomp_map_vars_internal, gomp_update): Likewise; unify and improve
	error message.
	* testsuite/libgomp.c-c++-common/target-present-2.c: Update for
	changed error message.
	* testsuite/libgomp.fortran/target-present-1.f90: Likewise.
	* testsuite/libgomp.fortran/target-present-2.f90: Likewise.
	* testsuite/libgomp.oacc-c-c++-common/present-1.c: Likewise.
	* testsuite/libgomp.c-c++-common/target-present-1.c: Likewise and
	extend testcase to check that data is copied when needed.
	* testsuite/libgomp.c-c++-common/target-present-3.c: Likewise.
	* testsuite/libgomp.fortran/target-present-3.f90: Likewise.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/defaultmap-4.c: Update scan-tree-dump.
	* c-c++-common/gomp/map-9.c: Likewise.
	* gfortran.dg/gomp/defaultmap-8.f90: Likewise.
	* gfortran.dg/gomp/map-11.f90: Likewise.
	* gfortran.dg/gomp/target-update-1.f90: Likewise.
	* gfortran.dg/gomp/map-12.f90: Likewise; also check original dump.
	* c-c++-common/gomp/map-6.c: Update dg-error and also check
	clause error with 'target (enter/exit) data'.