[OpenCL] Use function metadata to represent kernel attributes

This patch uses function metadata to represent reqd_work_group_size, work_group_size_hint and vector_type_hint kernel attributes and kernel argument info.

Differential Revision: http://reviews.llvm.org/D20979

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273425 91177308-0d34-0410-b5e6-96231b3b80d8