Resubmit r268719 - AMDGPU/SI: Add amdgpu_kernel calling convention. Part 2.

This was reverted in r268740 because of problems with corresponding Clang change.
Clang change was updated and resubmitted in r274220.

Check calling convention in AMDGPUMachineFunction::isKernel

This will be used for AMDGPU_HSA_KERNEL symbol type in output ELF.

Also, in the future unused non-kernels may be optimized.

Reviewers: tstellarAMD, arsenm

Subscribers: arsenm, joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D19917

llvm-svn: 274341