[dev.typeparams] import go2go changes to parse type parameters
This CL imports changes on the go2go branch to support parsing type
params, as well as the unsubmitted changes from CL 269300 to remove
support for parenthesize type parameter syntax.
Change-Id: I27ab942ce69eab62c2a1800f8f9661c4dcb233fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/270857
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>