[OpenCL] Setting constant address space for array initializers

Summary: Setting constant address space for global constants used for memcpy-initialization of arrays.

Patch by Alexey Sotkin.

Reviewers: bader, yaxunl, Anastasia

Subscribers: cfe-commits, AlexeySotkin

Differential Revision: https://reviews.llvm.org/D25305

llvm-svn: 285557