K2: Set correct containingFile in CheckerContext

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.

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostics/FileStructureElementDiagnosticRetriever.kt

@@ -121,7 +121,7 @@ internal object FileDiagnosticRetriever : FileStructureElementDiagnosticRetrieve
     ) {
         override fun visitFile(file: FirFile, data: Nothing?) {
             withAnnotationContainer(file) {
-                visitWithDeclaration(file) {
+                visitWithFile(file) {
                     file.annotations.forEach { it.accept(this, data) }
                     file.packageDirective.accept(this, data)
                     file.imports.forEach { it.accept(this, data) }