FIR checker: report REDUNDANT_LABEL_WARNING

Since many labels are not present in the FIR tree, this checker is
implemented as a syntax checker. Comparing with FE1.0, this change
reports some REDUNDANT_LABEL_WARNING that FE1.0 has missed, especially
LHS of assignments.

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

@@ -1353,6 +1353,10 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
         val MODIFIER_FORM_FOR_NON_BUILT_IN_SUSPEND by error<PsiElement>(PositioningStrategy.REFERENCED_NAME_BY_QUALIFIED)
         val RETURN_FOR_BUILT_IN_SUSPEND by error<KtReturnExpression>()
     }
+
+    val LABEL by object : DiagnosticGroup("label") {
+        val REDUNDANT_LABEL_WARNING by warning<KtLabelReferenceExpression>(PositioningStrategy.LABEL)
+    }
 }
 
 private val exposedVisibilityDiagnosticInit: DiagnosticBuilder.() -> Unit = {