go/format: fix //line corner case when formatting statements
The code formatting mechanism can be applied to partial Go code,
such as a list of statements. The statements are wrapped into a
function definition (to be parsed fine), and unwrapped after formatting.
When the statements contain //line annotations, it may fail,
because not all comments are flushed by the printer before the final '}'.
Formatting "\ta()\n//line :1" results in "\ta() }\n\n//line", which
is wrong.
Tweaked the wrapping/unwrapping code to make sure comments are flushed
before the '}'.
Fixes #11276
Change-Id: Id15c80279b0382ee9ed939cca1647f525c4929f5
Reviewed-on: https://go-review.googlesource.com/11282
Run-TryBot: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>