Omit @objcImpl from module interfaces

Module interfaces should not include the @objcImplementation attribute, member implementations that are redundant with the ObjC header, or anything that would be invalid in an ordinary extension (e.g. overridden initializers, stored Swift-only properties).

test/ModuleInterface/Inputs/objc_implementation/module.modulemap

@@ -0,0 +1,4 @@
+module objc_implementation {
+  header "objc_implementation.h"
+  export *
+}