Previously, it was working for all the checkers but the file-level
DeclarationChecker's because when execution comes to
DeclarationCheck::check for a file, `containingDeclarations` is empty
(it doesn't contain the file itself yet), thus some things that rely on
CheckerContext::containingFile don't work properly.
For example, SimpleDiagnosticsCollectorWithSuppress.report that
effectively doesn't collect diagnostics when the containingFile is null.
Denis.Zharkov
a71251d856