[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>

compiler/testData/diagnostics/tests/kt55181.kt

@@ -0,0 +1,5 @@
+// FIR_IDENTICAL
+
+fun main() {
+  throw <!TYPE_MISMATCH!>"str"<!>
+}