Return Unit.VALUE if Unit or Any is expected

StackValue.coerce will do its job in casting void to Unit correctly

 #KT-4212 Fixed

compiler/testData/codegen/box/unit/kt4212.kt

@@ -0,0 +1,20 @@
+fun foo(): Any? = bar()
+
+fun bar() {}
+
+fun baz(): Any? {
+    return bar()
+}
+
+fun quux(): Unit? = bar()
+
+fun box(): String {
+    foo()
+
+    if (foo() != Unit.VALUE) return "Fail 1"
+    if (foo() != bar()) return "Fail 2"
+    if (bar() != baz()) return "Fail 3"
+    if (baz() != quux()) return "Fail 4"
+
+    return "OK"
+}