compiler: Don't allow builtin function values.

According to the spec, http://golang.org/ref/spec#Built-in_functions:
    "built-in functions do not have standard Go types, so they can only
    appear in call expressions; they cannot be used as function values."
    
    Fixes golang/go#11570.
    
    Reviewed-on: https://go-review.googlesource.com/12543

From-SVN: r226448

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-9931f2c150e2da4b7d468db332823d8ef4fb8c34
+4c676d965c19b9c5d5e5049d0f8070502e9c27b0
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -3154,6 +3154,28 @@ Check_types_traverse::variable(Named_object* named_object)
 		     reason.c_str());
 	  var->clear_init();
 	}
+      else if (init != NULL
+               && init->func_expression() != NULL)
+        {
+          Named_object* no = init->func_expression()->named_object();
+          Function_type* fntype;
+          if (no->is_function())
+            fntype = no->func_value()->type();
+          else if (no->is_function_declaration())
+            fntype = no->func_declaration_value()->type();
+          else
+            go_unreachable();
+
+          // Builtin functions cannot be used as function values for variable
+          // initialization.
+          if (fntype->is_builtin())
+            {
+              error_at(init->location(),
+                       "invalid use of special builtin function %qs; "
+                       "must be called",
+                       no->message_name().c_str());
+            }
+        }
       else if (!var->is_used()
 	       && !var->is_global()
 	       && !var->is_parameter()