Introduce gimple_omp_single

Introduce gimple_omp_single

This corresponds to:
  [PATCH 42/89] Introduce gimple_omp_single
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01206.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/msg00807.html

gcc/
	* coretypes.h (gimple_omp_single): New typedef.
	(const_gimple_omp_single): New typedef.

	* gimple.h (gimple_build_omp_single): Return a gimple_omp_single
	rather than a plain gimple.
	(gimple_omp_single_set_clauses): Require a gimple_omp_single
	rather than a plain gimple.

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

	* gimple.c (gimple_build_omp_single): Return a gimple_omp_single
	rather than a plain gimple.

	* omp-low.c (scan_omp_single): Require a gimple_omp_single rather
	than a plain gimple.
	(scan_omp_1_stmt): Add checked cast to gimple_omp_single within
	GIMPLE_OMP_SINGLE case of switch statement.
	(lower_omp_single_simple): Require a gimple_omp_single rather
	than a plain gimple.
	(lower_omp_single_copy): Likewise.
	(lower_omp_single): Strengthen local "single_stmt" from gimple to
	gimple_omp_single.