[OPENCL] Fix wrongly vla error for OpenCL array.

Summary:
OpenCL should support array with const value size length, those const varibale in global and constant address space and variable in constant address space.

Reviewers: Anastasia, yaxunl, bader

Subscribers: bader, cfe-commits

Differential Revision: http://reviews.llvm.org/D20090

llvm-svn: 271971