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

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

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

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

Change-Id: Ib21eb7a87e11b77bcb2469985f9844964d35df57
Reviewed-on: https://go-review.googlesource.com/c/go/+/610540
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>