Enabled constepxr support for clang-cl.

[dutow]

No description provided.

[aantron]

Thanks, this looks good.

Do you know if clang-cl uses _CPPUNWIND to indicate that exceptions are supported and enabled? I would expect so, but would like to confirm.

Probably in a separate issue/PR, Better Enums should start running tests on clang-cl in AppVeyor. I haven't tried using clang-cl yet. Is there anything particular to watch out for when using it with CMake and/or in AppVeyor, if you have tried either?

[dutow]

Actually, I just started using clang-cl myself and encountered this problem in my first test project.

Using it with CMake is rather easy (you just have to specify the toolset with -TLLVM-vs2014), but I have no experience with AppVeyor.

I would expect _CPPUNWIND to work too, as it tries to simulate VS as much as possible, but I'll check it.

[aantron]

Thanks!