go/types, types2: factor type checking of func literals and generate go/types code

go/types, types2: factor type checking of func literals and generate go/types code

Move the code for type checking of function literals into
literals.go.

In go/types, the respective code is now generated from the types2 source.

Change-Id: Ic81ab3c0d3c66d99bc0f2e21d66bf9a896ef9375
Reviewed-on: https://go-review.googlesource.com/c/go/+/610996
Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>