Add test for exhaustive when with subject of Boolean! type

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

@@ -0,0 +1,21 @@
+// FIR_IDENTICAL
+// FILE: Provider.java
+
+public class Provider {
+    public static Boolean getCondition() {
+        return null;
+    }
+}
+
+// FILE: main.kt
+
+fun test_1(): Int = when (Provider.getCondition()) {
+    true -> 1
+    false -> 2
+}
+
+fun test_2(): Int = when (Provider.getCondition()) {
+    true -> 1
+    false -> 2
+    null -> 3
+}