[FIR] KT-55181: Ensure throw argument is Throwable

`TYPE_MISMATCH` in `throwJLException.fir.kt` appeared,
because in `throw Exn` the type of `Exn` is implicit Unit.
This is red code anyway.

^KT-55181 Fixed

Merge-request: KT-MR-8292
Merged-by: Nikolay Lunyak <Nikolay.Lunyak@jetbrains.com>

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosisCompilerTestFE10TestdataTestGenerated.java

@@ -501,6 +501,12 @@ public class DiagnosisCompilerTestFE10TestdataTestGenerated extends AbstractDiag
             runTest("compiler/testData/diagnostics/tests/kt54587_2.kt");
         }
 
+        @Test
+        @TestMetadata("kt55181.kt")
+        public void testKt55181() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/kt55181.kt");
+        }
+
         @Test
         @TestMetadata("kt55733.kt")
         public void testKt55733() throws Exception {