compiler: insert backend type conversion for closure func ptr

compiler: insert backend type conversion for closure func ptr

    In Func_expression::do_get_backend when creating the backend
    representation for a closure, create a backend type conversion to
    account for potential differences between the closure struct type
    (where the number of fields is dependent on the number of values
    referenced in the closure) and the generic function descriptor type
    (struct with single function pointer field).

    Reviewed-on: https://go-review.googlesource.com/49255

From-SVN: r250327