[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/delegation/Delegation_MultipleDelegates.fir.kt

@@ -0,0 +1,19 @@
+// !DIAGNOSTICS: -CONFLICTING_JVM_DECLARATIONS
+interface One {
+  public <!REDUNDANT_OPEN_IN_INTERFACE!>open<!> fun foo() : Int
+  private fun boo() = 10
+}
+interface Two {
+  public <!REDUNDANT_OPEN_IN_INTERFACE!>open<!> fun foo() : Int
+}
+
+interface OneImpl : One {
+  public override fun foo() = 1
+}
+interface TwoImpl : Two {
+  public override fun foo() = 2
+}
+
+<!CANNOT_INFER_VISIBILITY, MANY_INTERFACES_MEMBER_NOT_IMPLEMENTED!>class Test1<!>() : TwoImpl, OneImpl {}
+class Test2(a : One) : One by a, Two {}
+<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test3<!>(a : One, b : Two) : Two by b, One by a {}

compiler/testData/diagnostics/tests/delegation/Delegation_MultipleDelegates.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // !DIAGNOSTICS: -CONFLICTING_JVM_DECLARATIONS
 interface One {
   public <!REDUNDANT_OPEN_IN_INTERFACE!>open<!> fun foo() : Int