tree: introduce range adaptor for TREE_VEC

This patch implements a simple tree wrapper, named tree_vec_range, which
lets us idiomatically loop over all the elements of a TREE_VEC using a
C++11 range-based for loop:

  // v is a TREE_VEC
  for (tree e : tree_vec_range (v))
    ...

This is similar to the existing tree-based range adaptors ovl_range and
lkp_range added to the C++ FE in r12-340-g3307b9a07a3c51.

This patch also converts some existing loops over TREE_VEC within the
C++ FE to use tree_vec_range / range-for.

gcc/cp/ChangeLog:

	* constraint.cc (tsubst_parameter_mapping): Convert loop over
	TREE_VEC into a range-based for loop using tree_vec_range.
	* pt.cc (iterative_hash_template_arg): Likewise.
	(template_parms_level_to_args): Likewise.
	(deducible_template_args): Likewise.
	(check_undeduced_parms): Likewise.
	(dependent_type_p_r): Likewise.
	(value_dependent_expression_p) <case NONTYPE_ARGUMENT_PACK>:
	Likewise.
	(dependent_template_arg_p): Likewise.
	* tree.cc (cp_walk_subtrees) <case NONTYPE_ARGUMENT_PACK>:
	Likewise.

gcc/ChangeLog:

	* tree.h (TREE_VEC_BEGIN): Define.
	(TREE_VEC_END): Correct 'length' member access.
	(class tree_vec_range): Define.

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