[OpenCL] Pipe builtin functions

Summary:
Support for the pipe built-in functions for OpenCL 2.0.
The pipe builtin functions may have infinite kinds of element types, one approach
would be to just generate calls that would always use generic types such as void*.
This patch is based on bader's opencl support patch on SPIR-V branch.

Reviewers: Anastasia, pekka.jaaskelainen

Subscribers: keryell, bader, cfe-commits

Differential Revision: http://reviews.llvm.org/D15914

llvm-svn: 258773