cmd/compile: better error messages for use of ... with type parameters (dev.typeparams branch)

[griesemer]

For main.go:

package p
func _[T ...any]() {}

The compiler reports

main.go:2:10: ...any is not a type
main.go:2:10: invalid use of '...'

with go tool compile -G main.go.

The error messages should be better. ... should not be permitted for type parameters during parsing.

[gopherbot]

Change https://golang.org/cl/283475 mentions this issue: [dev.typeparams] cmd/compile/internal/types2: better error message for invalid ... use