testsuite: Introduce dg-ice.

This patch adds a new DejaGNU directive, dg-ice, as outlined in the
proposal here:
https://gcc.gnu.org/pipermail/gcc-patches/2020-July/550913.html

It means that it's expected that the compiler crashes with an internal
compiler error when compiling test with such a directive.

A minor optimization could be to use -pass-exit-codes and then check for
ICE_EXIT_CODE return code instead of using string match.

gcc/ChangeLog:

	* doc/sourcebuild.texi: Document dg-ice.

gcc/testsuite/ChangeLog:

	* lib/gcc-dg.exp (gcc-dg-test-1): Handle dg-ice.
	(cleanup-after-saved-dg-test): Reset expect_ice.
	* lib/prune.exp (prune_ices): New.
	* lib/target-supports-dg.exp (dg-ice): New.