[dev.typeparams] go/parser: support the ParseTypeParams mode

Support is added for parsing type parameters only if the ParseTypeParams
mode is set, otherwise emitting syntax errors for source code that is
invalid without type parameters.

Rather than have large conditional blocks switching between legacy
parser logic and new parser logic, effort is made to minimize special
handling for ParseTypeParams.

Change-Id: I243f6c4b9b8eb1313b838e8649b6cc1e5e8339ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/271218


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>