FIR: accept when(nothing) {} as exhaustive

FE1.0 accepts this but FIR current rejects it.

compiler/testData/diagnostics/tests/controlStructures/whenWithNothingTypedSubject.kt

@@ -0,0 +1,9 @@
+// FIR_IDENTICAL
+// !DIAGNOSTICS: -UNREACHABLE_CODE
+
+typealias MyNothing = Nothing
+
+fun foo(n: Nothing, n2: MyNothing) {
+    val a: Unit = when(n) {}
+    val b: Unit = when(n2) {}
+}

compiler/testData/diagnostics/tests/controlStructures/whenWithNothingTypedSubject.txt

@@ -0,0 +1,4 @@
+package
+
+public fun foo(/*0*/ n: kotlin.Nothing, /*1*/ n2: MyNothing /* = kotlin.Nothing */): kotlin.Unit
+public typealias MyNothing = kotlin.Nothing