[FIR] Consolidate SYNTAX and SYNTAX_WITH_MESSAGE diagnostics

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirSyntaxDiagnosticList.kt

@@ -13,8 +13,7 @@ import org.jetbrains.kotlin.fir.checkers.generator.diagnostics.model.DiagnosticL
 @OptIn(PrivateForInline::class)
 object SYNTAX_DIAGNOSTIC_LIST : DiagnosticList("FirSyntaxErrors") {
     val Syntax by object : DiagnosticGroup("Syntax") {
-        val SYNTAX by error<PsiElement>()
-        val SYNTAX_WITH_MESSAGE by error<PsiElement>() {
+        val SYNTAX by error<PsiElement> {
             parameter<String>("message")
         }
     }

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/coneDiagnosticToFirDiagnostic.kt

@@ -102,6 +102,7 @@ private fun ConeDiagnostic.toKtDiagnostic(
 
     is ConeSimpleDiagnostic -> when {
         source.kind is KtFakeSourceElementKind && source.kind != KtFakeSourceElementKind.ReferenceInAtomicQualifiedAccess -> null
+        kind == DiagnosticKind.Syntax -> FirSyntaxErrors.SYNTAX.createOn(qualifiedAccessSource ?: source, reason)
         else -> this.getFactory(source).createOn(qualifiedAccessSource ?: source)
     }
 
@@ -486,7 +487,6 @@ private val NewConstraintError.upperConeType: ConeKotlinType get() = upperType a
 
 private fun ConeSimpleDiagnostic.getFactory(source: KtSourceElement): KtDiagnosticFactory0 {
     return when (kind) {
-        DiagnosticKind.Syntax -> FirSyntaxErrors.SYNTAX
         DiagnosticKind.ReturnNotAllowed -> FirErrors.RETURN_NOT_ALLOWED
         DiagnosticKind.NotAFunctionLabel -> FirErrors.NOT_A_FUNCTION_LABEL
         DiagnosticKind.UnresolvedLabel -> FirErrors.UNRESOLVED_LABEL
@@ -532,6 +532,7 @@ private fun ConeSimpleDiagnostic.getFactory(source: KtSourceElement): KtDiagnost
         DiagnosticKind.UnresolvedSupertype,
         DiagnosticKind.UnresolvedExpandedType,
         DiagnosticKind.Other -> FirErrors.OTHER_ERROR
+        DiagnosticKind.Syntax -> error("Must not be called on `Syntax` because a message is required.")
     }
 }