[FIR] Missing NESTED_CLASS_ACCESSED_VIA_INSTANCE_REFERENCE

^KT-59433 Fixed

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/diagnostics/ConeDiagnostics.kt

@@ -272,6 +272,13 @@ class ConeTypeArgumentsForOuterClassWhenNestedReferencedError(source: KtSourceEl
     override val reason: String get() = "Type arguments for outer class are redundant when nested class is referenced"
 }
 
+class ConeNestedClassAccessedViaInstanceReference(
+    source: KtSourceElement,
+    val symbol: FirClassLikeSymbol<*>,
+) : ConeDiagnosticWithSource(source) {
+    override val reason: String get() = "Nested ${symbol.classId} accessed via instance reference"
+}
+
 class ConeNoTypeArgumentsOnRhsError(
     override val desiredCount: Int,
     override val symbol: FirClassLikeSymbol<*>