The new String methods use the new FormatFileInfo and
FormatDirEntry functions.

Fixes #54451

Change-Id: I414cdfc212ec3c316fb2734756d2117842a23631
Reviewed-on: https://go-review.googlesource.com/c/go/+/491175


Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>