[FIR] Add NOT_A_CLASS diagnostics

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -914,6 +914,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.NOT_A_CLASS) { firDiagnostic ->
+        NotAClassImpl(
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.OPT_IN_USAGE) { firDiagnostic ->
         OptInUsageImpl(
             firDiagnostic.a,

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -659,6 +659,10 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = RepeatedAnnotationWarning::class
     }
 
+    abstract class NotAClass : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = NotAClass::class
+    }
+
     abstract class OptInUsage : KtFirDiagnostic<PsiElement>() {
         override val diagnosticClass get() = OptInUsage::class
         abstract val optInMarkerFqName: FqName

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -790,6 +790,11 @@ internal class RepeatedAnnotationWarningImpl(
     override val token: ValidityToken,
 ) : KtFirDiagnostic.RepeatedAnnotationWarning(), KtAbstractFirDiagnostic<KtAnnotationEntry>
 
+internal class NotAClassImpl(
+    override val firDiagnostic: KtPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.NotAClass(), KtAbstractFirDiagnostic<PsiElement>
+
 internal class OptInUsageImpl(
     override val optInMarkerFqName: FqName,
     override val message: String,