[dev.typeparams] cmd/compile/internal/types2: move (remaining) type decls into their...

[dev.typeparams] cmd/compile/internal/types2: move (remaining) type decls into their own files (cleanup)

This change moves the type declarations and associated methods for
each of the remaining Type types into their respective files. Except
for import and comment adjustments, and receiver name adjustments for
the Underlying and String methods, no functional changes are made.

Change-Id: I3b9ccab3c85abea4852bacd28c2e47cec05c0bac
Reviewed-on: https://go-review.googlesource.com/c/go/+/332093
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>