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).

Change-Id: Id76b72f431099cfdf7343dc58e51dd70db67e359
Reviewed-on: https://go-review.googlesource.com/49255
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed