Introduce gimple_omp_task

Introduce gimple_omp_task

This corresponds to:
  [PATCH 41/89] Introduce gimple_omp_task
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01160.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK with expected changes due to renaming/updates to const handling.
> Please repost the final patch for archival purposes.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00806.html

gcc/
	* coretypes.h (gimple_omp_task): New typedef.
	(const_gimple_omp_task): New typedef.

	* gimple.h (gimple_build_omp_task): Return a gimple_omp_task
	rather than a plain gimple.

	* gimple-pretty-print.c (dump_gimple_omp_task): Require a
	gimple_omp_task rather than a plain gimple.
	(pp_gimple_stmt_1): Add checked cast to gimple_omp_task within
	GIMPLE_OMP_TASK case of switch statement.

	* gimple.c (gimple_build_omp_task): Return a gimple_omp_task
	rather than a plain gimple.

	* omp-low.c (finalize_task_copyfn): Require a gimple_omp_task
	rather than a plain gimple.
	(delete_omp_context): Add checked cast to gimple_omp_task.
	(scan_omp_task): Strengthen local "stmt" from gimple to
	gimple_omp_task.
	(expand_task_call): Require a gimple_omp_task rather than a plain
	gimple.
	(expand_omp_taskreg): Add checked cast to gimple_omp_task.
	(create_task_copyfn): Require a gimple_omp_task rather than a
	plain gimple.
	(lower_omp_taskreg): Add checked cast to gimple_omp_task.