Introduce gimple_debug and use it in a few places

Introduce gimple_debug and use it in a few places

This corresponds to:
  [PATCH 07/89] Introduce gimple_debug and use it in a few places
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01192.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> Same as prior patches.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00598.html

gcc/
	* coretypes.h (gimple_debug): New typedef.
	(const_gimple_debug): New typedef.

	* gimple.h (struct gimple_statement_debug): New subclass of
	gimple_statement_with_ops, adding the invariant that
	stmt->code == GIMPLE_DEBUG.
	(is_a_helper <gimple_statement_debug>::test): New.

	* gdbhooks.py (build_pretty_printer): Add gimple_debug and its
	variants, using the gimple printer.

	* gimple-pretty-print.c (dump_gimple_debug): Require a gimple_debug
	rather than just a gimple.
	* tree-inline.c (copy_debug_stmt): Likewise.

	* tree-inline.h (struct copy_body_data): Strengthen field
	"debug_stmts" from a vec<gimple> to a vec<gimple_debug>.

	* gimple.c (gimple_build_debug_bind_stat): Return a gimple_debug
	rather than just a gimple.
	(gimple_build_debug_source_bind_stat): Likewise.
	* gimple.h (gimple_build_debug_bind_stat): Likewise.
	(gimple_build_debug_source_bind_stat): Likewise.

	* tree-inline.c (remap_gimple_stmt): Update locals to be a
	gimple_debug.
	(maybe_move_debug_stmts_to_successors): Likewise.
	(copy_debug_stmts): Likewise.

	* gimple-pretty-print.c (pp_gimple_stmt_1): Add checked cast to
	gimple_debug in regions where a stmt is known to have code
	GIMPLE_DEBUG.