Introduce gimple_catch

Introduce gimple_catch

This corresponds to:
  [PATCH 25/89] Introduce gimple_catch
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01178.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK after fixing up the naming/const stuff as discussed for prior
> patches.
> That applies to 22-30. Make sure to take care of
> the pretty printers per Trevor's comments as well. He indicated those
> were missing in a couple of those patches.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00628.html

gcc/
	* coretypes.h (gimple_catch): New typedef.
	(const_gimple_catch): New typedef.

	* gimple-low.c (lower_try_catch): Add checked cast to gimple_catch.
	(gimple_try_catch_may_fallthru): Likewise.

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

	* gimple-walk.c (walk_gimple_op): Likewise.
	(walk_gimple_stmt): Likewise.

	* gimple.c (gimple_build_catch): Return a gimple_catch rather than
	a plain gimple.
	(gimple_copy): Add checked casts to gimple_catch within
	GIMPLE_CATCH case of switch statement, introducing new locals.

	* gimple.h (gimple_build_catch): Return a gimple_catch rather than
	a plain gimple.
	(gimple_catch_types_ptr): Require a gimple_catch rather than a
	plain gimple.
	(gimple_catch_handler_ptr): Likewise.
	(gimple_catch_handler): Likewise.
	(gimple_catch_set_types): Likewise.
	(gimple_catch_set_handler): Likewise.

	* omp-low.c (lower_omp_1): Add checked cast to gimple_catch within
	GIMPLE_CATCH case of switch statement.

	* tree-cfg.c (verify_gimple_in_seq_2): Likewise.
	(do_warn_unused_result): Likewise.

	* tree-eh.c (collect_finally_tree): Likewise.
	(replace_goto_queue_1): Likewise.
	(lower_catch): Convert local from gimple to gimple_catch.
	(refactor_eh_r): Add checked cast to gimple_catch within
	GIMPLE_CATCH case of switch statement.

	* tree-inline.c (remap_gimple_stmt): Likewise.
	(estimate_num_insns): Add checked cast to gimple_catch within
	GIMPLE_CATCH case of switch statement, introducing new local.