[dev.typeparams] go/types: don't modify Named.underlying in validType
This was fixed on dev.go2go in CL 240901, but accidentally omitted from
the merge.
Change-Id: I9020eb51dac4aa07d57c3de747d33ba84abb6386
Reviewed-on: https://go-review.googlesource.com/c/go/+/284254
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>