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/ExhaustiveNullable.kt

@@ -0,0 +1,14 @@
+enum class MyEnum {
+    A, B
+}
+
+fun foo(x: MyEnum?): Int {
+    val y: Int
+    // See KT-6046: y is always initialized
+    when (x) {
+        MyEnum.A -> y = 1
+        MyEnum.B -> y = 2
+        null -> y = 0
+    }
+    return y
+}