cmd/compile: better error messages for use of ... with type parameters

For `main.go`:
```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.