libgomp: Add omp_in_explicit_task support

This is pretty straightforward, if gomp_thread ()->task is NULL,
it can't be explicit task, otherwise if
gomp_thread ()->task->kind == GOMP_TASK_IMPLICIT, it is an implicit
task, otherwise explicit task.

2022-10-12  Jakub Jelinek  <jakub@redhat.com>

	* omp.h.in (omp_in_explicit_task): Declare.
	* omp_lib.h.in (omp_in_explicit_task): Likewise.
	* omp_lib.f90.in (omp_in_explicit_task): New interface.
	* libgomp.map (OMP_5.2): New symbol version, export
	omp_in_explicit_task and omp_in_explicit_task_.
	* task.c (omp_in_explicit_task): New function.
	* fortran.c (omp_in_explicit_task): Add ialias_redirect.
	(omp_in_explicit_task_): New function.
	* libgomp.texi (OpenMP 5.2): Mark omp_in_explicit_task as implemented.
	* testsuite/libgomp.c-c++-common/task-in-explicit-1.c: New test.
	* testsuite/libgomp.c-c++-common/task-in-explicit-2.c: New test.
	* testsuite/libgomp.c-c++-common/task-in-explicit-3.c: New test.