c++: streamline tf_qualifying_scope usage

c++: streamline tf_qualifying_scope usage

This patch introduces a convenience wrapper tsubst_scope for tsubst'ing
into a type with tf_qualifying_scope set, and makes suitable callers use
it instead of explicitly setting tf_qualifying_scope.  This patch also
makes tsubst_copy immediately delegate to tsubst for all type trees,
which allows tsubst_copy to be oblivious to the tf_qualifying_scope flag.

gcc/cp/ChangeLog:

	* pt.cc (tsubst_scope): Define.
	(tsubst_decl) <case USING_DECL>: Call tsubst_scope instead of
	calling tsubst_scope with tf_qualifying_scope set.
	(tsubst_qualified_id): Call tsubst_scope instead of
	calling tsubst with tf_qualifying_scope set.
	(tsubst_copy): Immediately delegate to tsubst for all TYPE_P
	trees.  Remove tf_qualifying_scope manipulation.
	<case SCOPE_REF>: Call tsubst_scope instead of calling
	tsubst with tf_qualifying_scope set.