Add an explicit diagnostic for when expect/actual modifiers are used in

an unsupported compilation

^KT-31281

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

@@ -3451,6 +3451,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.NOT_A_MULTIPLATFORM_COMPILATION) { firDiagnostic ->
+        NotAMultiplatformCompilationImpl(
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.INITIALIZER_REQUIRED_FOR_DESTRUCTURING_DECLARATION) { firDiagnostic ->
         InitializerRequiredForDestructuringDeclarationImpl(
             firDiagnostic as KtPsiDiagnostic,

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

@@ -2411,6 +2411,10 @@ sealed interface KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = ActualMissing::class
     }
 
+    interface NotAMultiplatformCompilation : KtFirDiagnostic<PsiElement> {
+        override val diagnosticClass get() = NotAMultiplatformCompilation::class
+    }
+
     interface InitializerRequiredForDestructuringDeclaration : KtFirDiagnostic<KtDestructuringDeclaration> {
         override val diagnosticClass get() = InitializerRequiredForDestructuringDeclaration::class
     }

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

@@ -2907,6 +2907,11 @@ internal class ActualMissingImpl(
     token: KtLifetimeToken,
 ) : KtAbstractFirDiagnostic<KtNamedDeclaration>(firDiagnostic, token), KtFirDiagnostic.ActualMissing
 
+internal class NotAMultiplatformCompilationImpl(
+    firDiagnostic: KtPsiDiagnostic,
+    token: KtLifetimeToken,
+) : KtAbstractFirDiagnostic<PsiElement>(firDiagnostic, token), KtFirDiagnostic.NotAMultiplatformCompilation
+
 internal class InitializerRequiredForDestructuringDeclarationImpl(
     firDiagnostic: KtPsiDiagnostic,
     token: KtLifetimeToken,