[modules] Re-enable the "ambiguous expansion of macro" warning.

Also update "test/Modules/macros.c" to test modified semantics:
-When there is an ambiguous macro, expand using the latest introduced version, not the first one.
-#undefs in submodules cause the macro to not be exported by that submodule, it doesn't cause
 undefining of macros in the translation unit that imported that submodule.
 This reduces macro namespace interference across modules.

llvm-svn: 178105