This patch:
- Adds SPIR-V backend's registered generator magic number to the emitted
binary. The magic number consists of the generator ID (43) and LLVM
major version.
- Adds SPIR-V version to the binary.
- Allows reading the expected (maximum supported) SPIR-V version from
the target triple.
- Uses VersionTuple for representing versions throughout the backend's
codebase.
- Registers v1.6 for spirv32 and spirv64 triple.
See more: https://github.com/KhronosGroup/SPIRV-Headers/commit/7d500c
This PR:
* adds __spirv_ builtins for existing instructions;
* fixes parsing of "syncscope" values in atomic instructions;
* fix a special case of binary header emision.
