[OpenCL] Add the default header file opencl-c.h for OpenCL C language

OpenCL has large number of "builtin" functions ("builtin" in the sense of OpenCL spec) which are defined in header files. To compile OpenCL kernels using these builtin functions, a header file is needed.

This header file is based on the Khronos implementation (https://github.com/KhronosGroup/SPIR/blob/spirv-1.0/lib/Headers/opencl.h) with heavy refactoring.

Differential Revision: http://reviews.llvm.org/D18369

llvm-svn: 271136