[OpenCL] Add Sema checks for OpenCL 2.0 block

Summary:
Add Sema checks for opencl 2.0 new features: Block.
This patch is partitioned from http://reviews.llvm.org/D16047

Reviewers: Anastasia

Subscribers: pekka.jaaskelainen, cfe-commits

Differential Revision: http://reviews.llvm.org/D17436