Before CL 36170, we identified all function bodies that needed to be
exported before writing any export data.

With CL 36170, we started identifying additional functions while
exporting function bodies. As a consequence, we cannot use a
range-based for loop for iterating over function bodies anymore.

Fixes #18895.

Change-Id: I9cbefa8d311ca8c9898c8272b2ac365976b02396
Reviewed-on: https://go-review.googlesource.com/45817
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>