go/build/constraint: add GoVersion

go/build/constraint: add GoVersion

For #57001, programs need to be able to deduce the Go version
implied by a given build constraint. GoVersion determines that,
by discarding all build tags other than Go versions and computing
the minimum Go version implied by the resulting expression.

For #59033.

Change-Id: Ifb1e7af2bdbdf172f82aa490c826c9b6ca5e824b
Reviewed-on: https://go-review.googlesource.com/c/go/+/476275
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>