Implementation of exhaustive whens in control flow analysis.

Now control flow analyzer knows when is exhaustive.
If despite of this all conditions are accidentally false, jump to error is generated.
A set of tests (diagnostic, control flow). #KT-5113 Fixed. #KT-6046 Fixed. #KT-1882 Fixed.

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

@@ -0,0 +1,16 @@
+// See KT-5113
+enum class E {
+    A,
+    B
+}
+
+class Outer(e: E) {
+    private val prop: Int
+    init {
+        when(e ) {
+            // When is exhaustive, property is always initialized
+            E.A -> prop = 1
+            E.B -> prop = 2
+        }
+    }
+}