go/types: add error reporting for 1.18 syntax if GoVersion is below 1.18

go/types: add error reporting for 1.18 syntax if GoVersion is below 1.18

This is a port of CL 344871 to go/types. Unlike the compiler, go/parser
is already always producing 1.18 syntax, so the effect of this CL is to
add some additional errors when Config.GoVersion is below 1.18.

This is a non-trivial port, both due to different error reporting APIs
and due to interacting with declaration syntax nodes, which differ
between go/ast and cmd/compile/internal/syntax.

Change-Id: I8003a014e6eec5e554c24e9a6cfc0548ec534834
Reviewed-on: https://go-review.googlesource.com/c/go/+/346433
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>