[OpenCL] Add predefined macros.

OpenCL spec requires __OPENCL_C_VERSION__ to be defined based on -cl-std option. This patch implements that.

The patch also defines __FAST_RELAXED_MATH__ based on -cl-fast-relaxed-math option.

Also fixed a test using -std=c99 for OpenCL program. Limit allowed language standard of OpenCL to be OpenCL standards.

Differential Revision: http://reviews.llvm.org/D19071

llvm-svn: 267590