cmd/api: use placeholder names for type parameters

Changing type parameter names is not a breaking API change, so we should
not include these names in the output of cmd/api. Instead print a
placeholder '$<index>' wherever type parameters are referenced.

This is valid for cmd/api as there is at most one type parameter list in
scope for any exported declaration. If we ever support method type
parameters, we'll need to revisit this syntax.

Change-Id: I7e677b1dab6ffeb0b79afefdb8d2580bef93891c
Reviewed-on: https://go-review.googlesource.com/c/go/+/355389


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>