go/types: generate builtins.go from types2 source

go/types: generate builtins.go from types2 source

Minor changes to types2.builtin.go to simplify automatic translation.

Added new conversion functions to generate_test.go to handle the
translation of builtins.go.

While at it, added additional helper functions to generate_test.go
and simplified some of the existing conversion functions.

This CL reduces the amount of code that needs to be maintained
manually by about 1000 LOC.

Change-Id: I1bd5c8eda0c0194a0b47e69882d2b987d91eef50
Reviewed-on: https://go-review.googlesource.com/c/go/+/562835
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>