Exhaustive when on boolean argument. A set of tests.

Compile-time constants are taken into account. #KT-3743 Fixed.

compiler/testData/diagnostics/tests/when/ExhaustiveBooleanNullable.kt

@@ -0,0 +1,9 @@
+// See also: KT-3743
+fun foo(arg: Boolean?): String {
+    // Must be exhaustive
+    return when(arg) {
+        true -> "truth"
+        false -> "falsehood"
+        null -> "unknown"
+    }
+}