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umar456 previously approved these changes Jun 10, 2020

* AF_CONSTEXPR expands to nothing if constexpr support is not available.
* Replace CONSTEXPR_DH with AF_CONSTEXPR and __DH__ in
  `src/backend/common/half.hpp`
* Removed AF_CONSTEXPR where it is invalid in half.hpp

* Adds the Zc:__cplusplus flag to cuda builds for MSVC if the flag is available.
the cuda_fp16 header does not define the default constructor for __half as
"= default" and that prevents the __half struct to be used in a constexpr
expression
* For older versions of MSVC we define the __cplusplus macro before and
after the inclusion of cuda_fp16.h header.
* Define the AF_CONSTEXPR macro for NVRTC compilation

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