go/internal/gcimporter: indexed format imports for type parameters aliases

Add support for importing a new 'B' tag for type parameters aliases
in the indexed data format.

Updates #68778

Change-Id: I3bd82870d4c4619a3771de30baf6d54f6ee5959e
Reviewed-on: https://go-review.googlesource.com/c/go/+/604635


Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>