[OpenCL] Mark group functions as convergent in opencl-c.h

Certain OpenCL builtin functions are supposed to be executed by all threads in a work group or sub group. Such functions should not be made divergent during transformation. It makes sense to mark them with convergent attribute.

The adding of convergent attribute is based on Ettore Speziale's work and the original proposal and patch can be found at https://www.mail-archive.com/cfe-commits@lists.llvm.org/msg22271.html.

Differential Revision: https://reviews.llvm.org/D25343

llvm-svn: 285725