This is mostly Russ's https://golang.org/cl/12145 but with some extra fixes to
account for the fact that function declarations without implementations now
break shared libraries, and including my test case.

Fixes #11480.

Change-Id: Iabdc2934a0378e5025e4e7affadb535eaef2c8f1
Reviewed-on: https://go-review.googlesource.com/12340Reviewed-by: Ian Lance Taylor <iant@golang.org>