cmd/api: fix panic on exported basic type aliases

cmd/api: fix panic on exported basic type aliases

The order of emitting named type and type aliases in the `Walker`'s
`emitType` function is inverted. When the type alias references a basic
type, this causes a panic as the type assertion on `*types.Named` fails.
This change reorders the logic such that type aliases are emitted prior
to this type assertion.

Fixes #64958

Change-Id: I52dbe13999978912ded788d9cf4948103869bcfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/554076
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>