Introduce gimple_omp_sections

Introduce gimple_omp_sections

This corresponds to:
  [PATCH 45/89] Introduce gimple_omp_sections
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01188.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/msg00821.html

gcc/
	* coretypes.h (gimple_omp_sections): New typedef.
	(const_gimple_omp_sections): New typedef.

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

	* gimple.c (gimple_build_omp_sections): Return a
	gimple_omp_sections rather than a plain gimple.

	* gimple.h (gimple_build_omp_sections): Return a
	gimple_omp_sections rather than a plain gimple.

	* omp-low.c (scan_omp_sections): Require a gimple_omp_sections
	rather than a plain gimple.
	(scan_omp_1_stmt): Add checked cast to gimple_omp_sections within
	GIMPLE_OMP_SECTIONS case of switch statement.
	(expand_omp_sections): Strengthen local "sections_stmt" from gimple
	to gimple_omp_sections.
	(lower_omp_sections): Likewise for "stmt".