c++: Hash placeholder constraint in ctp_hasher

c++: Hash placeholder constraint in ctp_hasher

This patch addresses a difference between the hash function and the equality
function for canonical types of template parameters (ctp_hasher). The equality
function uses comptypes (typeck.cc) (with COMPARE_STRUCTURAL) and checks
constraint equality for two auto nodes (typeck.cc:1586), while the hash
function ignores it (pt.cc:4528). This leads to hash collisions that can be
avoided by using `hash_placeholder_constraint` (constraint.cc:1150).

Note that due to the proper handling of hash collisions (hash-table.h:1059),
there is no test case that can distinguish the current implementation from the
proposed one.

	* constraint.cc (hash_placeholder_constraint): Rename to
	iterative_hash_placeholder_constraint.
	(iterative_hash_placeholder_constraint): Rename from
	hash_placeholder_constraint and add the initial val argument.
	* cp-tree.h (hash_placeholder_constraint): Rename to
	iterative_hash_placeholder_constraint.
	(iterative_hash_placeholder_constraint): Renamed from
	hash_placeholder_constraint and add the initial val argument.
	* pt.cc (struct ctp_hasher): Updated to use
	iterative_hash_placeholder_constraint in the case of a valid placeholder
	constraint.
	(auto_hash::hash): Reflect the renaming of hash_placeholder_constraint to
	iterative_hash_placeholder_constraint.