Adding test

tests/cases/conformance/types/union/unionTypeReduction.ts

@@ -0,0 +1,16 @@
+interface I2 {
+    (): number;
+    (q): boolean;
+}
+
+interface I3 {
+    (): number;
+}
+
+var i2: I2, i3: I3;
+
+var e1: I2 | I3;
+var e2 = i2 || i3;  // Type of e2 immediately reduced to I3
+
+var r1 = e1();  // Type of e1 reduced to I3 upon accessing property or signature
+var r2 = e2();