spec: clarify rules for receiver base types

The spec currently provides a syntactic rule for receiver base types,
and a strict reading of those rules prohibits the use of type aliases
referring to pointer types as receiver types.

This strict interpretation breaks an assumed rule for aliases, which
is that a type literal can always be replaced by an alias denoting
that literal.

Furthermore, cmd/compile always accepted this new formulation of the
receiver type rules and so this change will simply validate what has
been implemented all along.

Fixes #27995.

Change-Id: I032289c926a4f070d6f7795431d86635fe64d907
Reviewed-on: https://go-review.googlesource.com/c/142757


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>