Add "A module cannot have multiple default exports." message for multiple "default" exports

tests/cases/conformance/es6/modules/defaultExportWithOverloads01.ts

@@ -0,0 +1,7 @@
+// @module: commonjs
+// @target: ES5
+
+export default function f();
+export default function f(x: string);
+export default function f(...args: any[]) {
+}

tests/cases/conformance/es6/modules/multipleDefaultExports03.ts

@@ -0,0 +1,8 @@
+// @module: commonjs
+// @target: ES5
+
+export default class C {
+}
+
+export default class C {
+}

tests/cases/conformance/es6/modules/multipleDefaultExports04.ts

@@ -0,0 +1,8 @@
+// @module: commonjs
+// @target: ES5
+
+export default function f() {
+}
+
+export default function f() {
+}