Support empty if-statements

Type-check "if (...) ;" to Unit, report "implicit cast to Unit", propagate data
flow info out of its condition

 #KT-2478 Fixed

compiler/testData/diagnostics/tests/controlStructures/emptyIf.kt

@@ -0,0 +1,16 @@
+fun foo(x: Unit) = x
+
+fun test() {
+    if (false);
+    if (true);
+
+    val x = <!IMPLICIT_CAST_TO_UNIT_OR_ANY!>if (false)<!>;
+    foo(x)
+
+    val y: Unit = if (false);
+    foo(y)
+
+    foo({if (1==1);}())
+
+    return if (true);
+}