One of the platforms on which we do regular builds has some system headers that are not compatible with C++14. As a result, we can't compile any code (including test-suite tests) if we leave the default as is. So what we do is set the CMake option that allows us to set clang's default language level (we set it to gnu++11 in our case). However, this causes failures in all of the lit tests attached.
This patch simply adds the -std=gnu++14 option to match the typical default for clang that the test cases rely on.
I've tried to add people that have added/modified the tests as reviewers to this patch - so I am sorry about the very long list of reviewers. Please let me know if this is an acceptable change.