go/ast/astutil: don't panic trying to copy FuncType

In gh-1134, I argued in favour of extending CopyExpr to use reflection
to handle all types, under the assumption that we missed many types.
I have now decided against that, for two reasons:

1. There weren't many unhandled types left
2. We cannot copy comments because of limitations in go/ast, and it
seems better to refuse generating suggested fixes than to swallow
comments. This is a general problem, but function literals are
particularly likely to contain comments.

Thus, we just add FuncType to the list of types we cannot copy. The
effect is that several checks in the quickfix package will not generate
suggested fixes in some cases.

Closes gh-1134

(cherry picked from commit 8af8898)

Author: dominikh

go/ast/astutil/util.go

@@ -218,8 +218,8 @@ func CopyExpr(node ast.Expr) (ast.Expr, bool) {
 	case *ast.StructType:
 		cp := *node
 		return &cp, true
-	case *ast.FuncLit:
-		// TODO(dh): implement copying of function literals.
+	case *ast.FuncLit, *ast.FuncType:
+		// TODO(dh): implement copying of function literals and types.
 		return nil, false
 	case *ast.ChanType:
 		var ok bool