If the char class is 0x0-0x10ffff we mistakenly would String that to `[^]`,
which is not a valid regex.

Fixes #31807

Change-Id: I9ceeaddc28b67b8e1de12b6703bcb124cc784556
Reviewed-on: https://go-review.googlesource.com/c/go/+/175679Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>