cmd/doc, go/doc: add basic support for generic code

Update cmd/doc and go/doc for the generics, by adding handling for type
parameters and the new embedded interface elements.

Specifically:
 - Format type parameters when summarizing type and function nodes.
 - Find the origin type name for instantiation expressions, so that
   methods are associated with generic type declarations.
 - Generalize the handling of embedding 'error' in interfaces to
   arbitrary predeclared types.
 - Keep embedded type literals.
 - Update filtering to descend into embedded type literals.

Also add "any" to the list of predeclared types.

Updates #49210

Change-Id: I6ea82869f19c3cdbc3c842f01581c8fc7e1c2ee7
Reviewed-on: https://go-review.googlesource.com/c/go/+/359778


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>