c++: reduce unnecessary tree_common

c++: reduce unnecessary tree_common

Lewis' r15-5067 fixing the marking of TRAIT_EXPR led me to compare some
other front-end type definitions to their marking in cp_common_init_ts; it
seems we can change tree_common to something smaller in several cases, to
match how they are marked.

gcc/cp/ChangeLog:

	* cp-tree.h (struct ptrmem_cst): Change tree_common to tree_typed.
	(struct tree_trait_expr): Likewise.
	(struct tree_static_assert): Change tree_common to tree_base.
	(struct tree_argument_pack_select): Likewise.