[NFC] github: Add stubs for IRGen owners

Author: AnthonyLatsis

.github/CODEOWNERS

@@ -99,3 +99,8 @@ utils/*windows*       @compnerd
 # TODO: /include/swift/SILOptimizer/
 # TODO: /lib/SILOptimizer/
 # TODO: /*test/SILOptimizer/
+
+# IRGen
+# TODO: /include/swift/IRGen/
+# TODO: /lib/IRGen/
+# TODO: /*test/IRGen/