regard TemplateLiteral as discriminant property (#44569)

tests/cases/conformance/types/union/discriminatedUnionTypes3.ts

@@ -0,0 +1,19 @@
+// @strict: true
+
+// Repro from #44435
+
+type Correct = {
+	code: string
+	property: true
+	err: undefined
+}
+type Err = {
+	err: `${string} is wrong!`
+}
+type SomeReturnType = Correct | Err;
+
+const example: SomeReturnType = {} as SomeReturnType;
+
+if (example.err === undefined) {
+	example.property;  // true
+}