OpenCL: CL2.0 atomic types

OpenCL C Spec v2.0 Section 6.13.11

- Made c11 _Atomic being not accepted for OpenCL

- Implemented CL2.0 atomics by aliasing them to the corresponding c11 atomic types using implicit typedef

- Added diagnostics for atomics Khronos extension enabling

llvm-svn: 232631
6 files changed