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/ExhaustiveInitialization.kt

@@ -0,0 +1,15 @@
+enum class Direction {
+    NORTH, SOUTH, WEST, EAST
+}
+
+fun foo(dir: Direction): Int {
+    val res: Int
+    // See KT-6046: res is always initialized
+    when (dir) {
+        Direction.NORTH -> res = 1
+        Direction.SOUTH -> res = 2
+        Direction.WEST  -> res = 3
+        Direction.EAST  -> res = 4
+    }
+    return res
+}