ir_function_signature: Add method to get the function owning a signature

ir_function_signature: Add method to get the function owning a signature

There is no setter function, the getter returns a constant pointer,
and ir_function_signature::_function is private for a reason.  The
only way to make a connection between a function and function
signature is via ir_function::add_signature.  This helps ensure that
certain invariants (i.e., a function signature is in the list of
signatures for its _function) are met.