When printing types for the OpenCL kernel metadata, use the PrintingPolicy.

This allows 'half' to be printed as 'half' and not as '__fp16'.

Patch by Fraser Cormack!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205624 91177308-0d34-0410-b5e6-96231b3b80d8