AMDGPU/SI: Add amdgpu_kernel calling convention. Part 1.
This will be used for AMDGPU_HSA_KERNEL symbol type in output ELF.
Also, in the future unused non-kernels may be optimized.
For now, also accept SPIR_KERNEL for HCC frontend.
Also, add bitcode compatibility tests for missing calling conventions
except AVR_BUILTIN which doesn't have parse code.
Reviewers: tstellarAMD, arsenm
Subscribers: arsenm, joker.eph, llvm-commits