Blender Git Loki

Fix: Link drag search missing field nodes from function nodes

When dragging from the inputs of function nodes, other function
nodes wouldn't connect, because their socket declaration field types
weren't set correctly. Instead, they relied on code properly checking
the *node* declaration's `is_function_node()` method. However,
that increases complexity and requires passing the node instead of
just the socket in more places. Instead, set the proper field types
in the socket declaration during building.

Differential Revision: https://developer.blender.org/D13600