Introduce gimple_eh_filter

Introduce gimple_eh_filter

This corresponds to:
  [PATCH 26/89] Introduce gimple_eh_filter
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01220.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_eh_filter): New typedef.
	(const_gimple_eh_filter): New typedef.

	* gimple.h (gimple_build_eh_filter): Return a gimple_eh_filter
	rather than a plain gimple.

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

	* gimple.c (gimple_build_eh_filter): Return a gimple_eh_filter
	rather than a plain gimple.