add a hash_set based on hash_table

This allows us to replace the usage of pointer_set outside of
pointer_map with a nicer interface.

gcc/ada/

	* gcc-interface/trans.c: Use hash_set instead of pointer_set.

gcc/c-family/

	* c-gimplify.c: Use hash_set instead of pointer_set.

gcc/c/

	* c-decl.c: Use hash_set instead of pointer_set.

gcc/cp/

	* class.c, cp-gimplify.c, cp-tree.h, decl.c, decl2.c, error.c,
	method.c, name-lookup.c, pt.c, semantics.c, tree.c: Use hash_set
	instead of pointer_set.

gcc/fortran/

	* openmp.c, trans-decl.c: Use hash_set instead of pointer_set.

gcc/

	* hash-set.h: new File.
	* cfgexpand.c, cfgloop.c, cgraph.c, cgraphbuild.c, cgraphunit.c,
	cprop.c, cse.c, gimple-walk.c, gimple-walk.h, gimplify.c, godump.c,
	ipa-devirt.c, ipa-pure-const.c, ipa-visibility.c, ipa.c, lto-cgraph.c,
	lto-streamer-out.c, stmt.c, tree-cfg.c, tree-core.h, tree-eh.c,
	tree-inline.c, tree-inline.h, tree-nested.c, tree-pretty-print.c,
	tree-ssa-loop-niter.c, tree-ssa-phiopt.c, tree-ssa-threadedge.c,
	tree-ssa-uninit.c, tree.c, tree.h, value-prof.c, varasm.c,
	varpool.c: Use hash_set instead of pointer_set.

gcc/lto/

	* lto-partition.c, lto-partition.h: Use hash_set instead of
	pointer_set.

From-SVN: r213516