[dev.typeparams] go/types: add some missing APIs for the importer
This is a partial port of CL 319930, containing only changes to
go/types. Importer changes will be made in a separate CL.
The TypeParams APIs are left unexported for now; they will be exported
when they are needed.
Change-Id: I74bd246d4c174cb38f8360d921c733fa03900eab
Reviewed-on: https://go-review.googlesource.com/c/go/+/335143
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>