[release-branch.go1.9] cmd/compile: simplify "missing function body" error message

[release-branch.go1.9] cmd/compile: simplify "missing function body" error message

Fixes #21747.

Change-Id: I6a68370be3b7510ce364ddd1e41a1d767ce3a67f
Reviewed-on: https://go-review.googlesource.com/61311
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-on: https://go-review.googlesource.com/70972
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>