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

gcc/go/gofrontend/MERGE

-c49ddc84f3ce89310585aad23ab6e51ef5523748
+3d9ff9bc339942922f1be3bef07c6fe2978ad81a
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -1204,7 +1204,14 @@ Func_expression::do_get_backend(Translate_context* context)
   // expression.  It is a pointer to a struct whose first field points
   // to the function code and whose remaining fields are the addresses
   // of the closed-over variables.
-  return this->closure_->get_backend(context);
+  Bexpression *bexpr = this->closure_->get_backend(context);
+
+  // Introduce a backend type conversion, to account for any differences
+  // between the argument type (function descriptor, struct with a
+  // single field) and the closure (struct with multiple fields).
+  Gogo* gogo = context->gogo();
+  Btype *btype = this->type()->get_backend(gogo);
+  return gogo->backend()->convert_expression(btype, bexpr, this->location());
 }
 
 // Ast dump for function.