cmd/compile: use 'not defined' rather than 'unnamed' in error message

A receiver type may have an (alias type) name and thus be 'named'
even though the name doesn't refer to a defined type. Adjust the
error message to make this clearer.

Change-Id: I969bf8d1ba3db8820f67f6ecd6d5cfe564c5b80d
Reviewed-on: https://go-review.googlesource.com/112638


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>