cmd/gc: fix error message for type errors involving conversions.

Fixes #3818.

R=golang-dev, rsc, r
CC=golang-dev, remy
https://golang.org/cl/6352106

src/cmd/gc/subr.c

@@ -1384,6 +1384,7 @@ assignconv(Node *n, Type *t, char *context)
 	r->type = t;
 	r->typecheck = 1;
 	r->implicit = 1;
+	r->orig = n;
 	return r;
 }