Added test from #6278.

Author: DanielRosenwasser

tests/cases/conformance/types/stringLiteral/stringLiteralTypesOverloads05.ts

@@ -0,0 +1,13 @@
+// @declaration: true
+
+interface Animal { animal: {} };
+interface Dog extends Animal { dog: {} }
+interface Cat extends Animal { cat: {} }
+interface Moose extends Animal { moose: {} }
+
+function doThing(x: "dog"): Dog;
+function doThing(x: "cat"): Cat;
+function doThing(x: string): Animal;
+function doThing(x: string, y?: string): Moose {
+    return undefined;
+}