[OpenCL] Added half type literal with suffix h.

OpenCL Extension v1.2 s9.5 allows half precision floating point
type literals with suffices h or H when cl_khr_fp16 is enabled.

Example:  half x = 1.0h;

Patch by Liu Yaxun (Sam)!

Differential Revision: http://reviews.llvm.org/D16865

llvm-svn: 261084