Support GTY((cache)) on hash_map.

Support GTY((cache)) on hash_map.

gcc/
	* hash-traits.h (ggc_remove): Add ggc_maybe_mx member function.
	(ggc_cache_remove): Override it instead of ggc_mx.
	* hash-table.h (gt_ggc_mx): Call it instead of ggc_mx.
	(gt_cleare_cache): Call ggc_mx instead of gt_ggc_mx.
	* hash-map-traits.h (simple_hashmap_traits): Add maybe_mx member.
	(simple_cache_map_traits): Override maybe_mx.
	* hash-map.h (hash_entry): Add ggc_maybe_mx and keep_cache_entry.
	(hash_map): Friend gt_cleare_cache.
	(gt_cleare_cache): New.
	* tree.h (tree_cache_traits): New hash_map traits class.
	(tree_cache_map): New typedef.
gcc/cp/
	* decl.c (decomp_type_table): Use tree_cache_map.
	* init.c (nsdmi_inst): Likewise.
	* pt.c (defarg_ints): Likewise.
	* cp-objcp-common.c (cp_get_debug_type): Likewise.

From-SVN: r254731