Modules hide macro definitions by default, so that silly things like

include guards don't show up as macro definitions in every translation
unit that imports a module. Macro definitions can, however, be
exported with the intentionally-ugly #__export_macro__
directive. Implement this feature by not even bothering to serialize
non-exported macros to a module, because clients of that module need
not (should not) know that these macros even exist.

llvm-svn: 138943