For modules, all macros that aren't include guards are implicitly

For modules, all macros that aren't include guards are implicitly
public. Add a __private_macro__ directive to hide a macro, similar to
the __module_private__ declaration specifier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142188 91177308-0d34-0410-b5e6-96231b3b80d8