Printing a function value is nearly useless outside of debugging, but
can occur by mistake when one forgets to call it. Diagnose this.

I did this myself just the other day and it arose in cl/14031.
Easy to fix and seems worthwhile.

Fixes #12295.

Change-Id: Ice125a84559f0394f7fa7272b5d31ae602b07f83
Reviewed-on: https://go-review.googlesource.com/14122Reviewed-by: Andrew Gerrand <adg@golang.org>