This should is preparatory cleanup to make it easier to use separate
types to represent each kind of Go type, rather than a single omnibus
Type struct with heavily overloaded fields.

Also, add TODO comments marking assignments that change an existing
Type's kind, as they need to be removed before we can factor Type.

Change-Id: If4b551fdea4ae045b10b1a3de2ee98f5cf32a517
Reviewed-on: https://go-review.googlesource.com/20494Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>