C++1y: Update __cplusplus to temporary value 201305L to allow detection of provisional C++1y support.
Add __has_feature and __has_extension checks for C++1y features (based on the provisional names from
the C++ features study group), and update documentation to match.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181342 91177308-0d34-0410-b5e6-96231b3b80d8