Make pointer_query cache a private member.

Make pointer_query cache a private member.

gcc/ChangeLog:

	* gimple-ssa-warn-access.cc (pass_waccess::pass_waccess): Remove
	pointer_query cache.
	* pointer-query.cc (pointer_query::pointer_query): Remove cache
	argument.  Zero-initialize new cache member.
	(pointer_query::get_ref): Replace cache pointer with direct access.
	(pointer_query::put_ref): Same.
	(pointer_query::flush_cache): Same.
	(pointer_query::dump): Same.
	* pointer-query.h (class pointer_query): Remove cache argument from
	ctor.  Change cache pointer to cache subobject member.
	* tree-ssa-strlen.cc: Remove pointer_query cache.