regtests can use Await to wait for diagnostic expectations. But sometimes
it is useful (or more robust) to then look at the specific diagnostics.
This change introduces env.DiagnosticsFor, which returns the current
diagnostics for a file.
Change-Id: Iea35d28f6679289795bc853f156aae351279b205
Reviewed-on: https://go-review.googlesource.com/c/tools/+/236837
Run-TryBot: Peter Weinberger <pjw@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>