go/types, types2: update error message for untyped composite literals that can't be typed

[mrkfrmn]

Moving from CL 794088:

var (
       _ = { /* ERROR "missing type in composite literal" */ }
       _ []int = {}
       _ struct{} = {}
      _ [10]byte = {1, 2, 3, 9: 10}
       _ map[string]int = {"foo": 0, "bar": 1}
)

We might want the error to say missing type for composite literal rather than in.