Only warn if the method is missing for all types.
Fixesgolang/go#30971
Change-Id: I94169ad3266f68ca20378a8dc5538aed2541a773
Reviewed-on: https://go-review.googlesource.com/c/tools/+/168803
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>