[FIR] Report all Visibilities.Unknown in FirOverrideChecker

Check all members for `Visibility.Unknown`,
otherwise we miss them when they come
from supertypes. This is the reason why
the FP intellij build failed with a
cryptic stacktrace instead of a
human-readable diagnostic.

Also, do report the diagnostic at all
cases of `Visibilities.Unknown`. Turns
out, there are no "simple to reason
about" situations here :(

Also, an interesting detail:
`retrieveDirectOverriddenOf` returns an
empty list for intersection overrides.
But this doesn't seem to break anything...

Replacing `CANNOT_INFER_VISIBILITY`'s
type `KtDeclaration` with
`PsiNameIdentifierOwner` and the related
changes in `PositioningStrategies`
were needed to prevent an exception saying that
`PsiClassImpl` is not a subtype of
`KtDeclaration`.

compiler/frontend.common-psi/src/org/jetbrains/kotlin/diagnostics/PositioningStrategies.kt

@@ -22,7 +22,7 @@ import org.jetbrains.kotlin.utils.addToStdlib.safeAs
 import org.jetbrains.kotlin.utils.sure
 
 object PositioningStrategies {
-    open class DeclarationHeader<T : KtDeclaration> : PositioningStrategy<T>() {
+    open class DeclarationHeader<T : PsiElement> : PositioningStrategy<T>() {
         override fun isValid(element: T): Boolean {
             if (element is KtNamedDeclaration &&
                 element !is KtObjectDeclaration &&
@@ -142,8 +142,8 @@ object PositioningStrategies {
     }
 
     @JvmField
-    val DECLARATION_NAME: PositioningStrategy<KtNamedDeclaration> = object : DeclarationHeader<KtNamedDeclaration>() {
-        override fun mark(element: KtNamedDeclaration): List<TextRange> {
+    val DECLARATION_NAME: PositioningStrategy<PsiNameIdentifierOwner> = object : DeclarationHeader<PsiNameIdentifierOwner>() {
+        override fun mark(element: PsiNameIdentifierOwner): List<TextRange> {
             val nameIdentifier = element.nameIdentifier
             if (nameIdentifier != null) {
                 if (element is KtClassOrObject) {