AMDGPU: Set amdgpu_kernel calling convention for OpenCL kernels.

Summary:
Summary:
Change Clang calling convention SpirKernel to OpenCLKernel.
Set calling convention OpenCLKernel for amdgcn as well.
Add virtual method .getOpenCLKernelCallingConv() to TargetCodeGenInfo
and use it to set target calling convention for AMDGPU and SPIR.
Update tests.

Reviewers: rsmith, tstellarAMD, Anastasia, yaxunl

Subscribers: kzhuravl, cfe-commits

Differential Revision: http://reviews.llvm.org/D21367

llvm-svn: 274220