[Feature Request] Use latest version spirv headers from Khronos repository.

[bader]

Currently we have copy of spirv.hpp file that defines opcodes of SPIR-V instructions. Khronos now provides the latest official version of this header file in the separate repository.

We should integrate Khronos headers into our build process, so we can use the latest version in llvm and clang projects, since the version we are using at the moment is already outdated.

https://github.com/KhronosGroup/SPIRV-Headers

[pierremoreau]

Awesome, finally a repo for SPIR-V headers!

To be able to use the 1.0 rev 4 version, and by extension 1.1, the current code needs to drop its usage of OpForward and LinkageTypeInternal, or regenerate the official headers to add those removed enum values.