[cfe-commits] [libc++] Switching to 'double underscore' notation for has_feature.

[Jean-Daniel Dupas]

Hi,

clang recently gained support for double underscore variant of the features in __has_feature test (the same way attributes support a double underscore variant).
This change provide a way to safely use __has_feature in system headers without conflicting with potential user defined macros.

As libc++ headers are system headers, I suggest we switch to using the double underscore variant. 
A problem I see is that applying this change will break compatibility with older clang version. 
If this is not an issue, I attached a patch that performs this change.

Thanks

-- Jean-Daniel



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