unordered: Rename __construct_node_hash() to allow forwarding, NFC

Rename the version of __construct_node() that takes a hash as an
argument to __construct_node_hash(), and use perfect-forwarding when
Rvalue references are available.  The primary motivation is to allow
other types through, since unordered_map's value_type is different from
__hash_table's value_type -- a follow-up will take advantage of this --
but the rename is general "goodness".

There should be no functionality change here (aside from enabling the
follow-up).

llvm-svn: 258511