cmd/fix: use reflect.TypeFor for known types

cmd/fix: use reflect.TypeFor for known types

For #60088

Change-Id: Id19435e864bcfd2adbb1492db3f8cdf2ee3c915e
Reviewed-on: https://go-review.googlesource.com/c/go/+/515175
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: shuang cui <imcusg@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>