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/cfg/controlStructures/whenExhaustive.kt

@@ -0,0 +1,6 @@
+fun foo(flag: Boolean): Int {
+    when (flag) {
+        true -> return 1
+        false -> return 0
+    }
+}