[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/testData/diagnostics/tests/override/FakeOverrideDifferentDeclarationSignatures.fir.kt

@@ -0,0 +1,11 @@
+interface A {
+    fun f(): String = "string"
+}
+
+open class B {
+    open fun f(): CharSequence = "charSequence"
+}
+
+<!CANNOT_INFER_VISIBILITY, MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class C<!> : B(), A
+
+val obj: A = <!CANNOT_INFER_VISIBILITY, MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>object<!> : B(), A {}