[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/regressions/kt302.fir.kt

@@ -0,0 +1,13 @@
+// KT-302 Report an error when inheriting many implementations of the same member
+
+package kt302
+
+interface A {
+    open fun foo() {}
+}
+
+interface B {
+    open fun foo() {}
+}
+
+<!CANNOT_INFER_VISIBILITY, MANY_INTERFACES_MEMBER_NOT_IMPLEMENTED!>class C<!> : A, B {} //should be error here

compiler/testData/diagnostics/tests/regressions/kt302.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // KT-302 Report an error when inheriting many implementations of the same member
 
 package kt302