Apply existing checks to C++1Z (has_feature_cxx0x); NFC

This applies the existing checks in has_feature_cxx0x to -std=c++1z.
In addition, references to C++1y are updated to refer to C++14

No functional change. Testing of __has_feature for C++1z features
is not added in this change.

Patch by Hubert Tong! 

llvm-svn: 237992