[OpenCL] Enable unroll hint for OpenCL 1.x.

Summary: Although the feature was introduced only in OpenCL C v2.0 spec., it's useful for OpenCL 1.x too and doesn't require HW support.

Reviewers: Anastasia

Subscribers: yaxunl, cfe-commits, bader

Differential Revision: https://reviews.llvm.org/D27453

llvm-svn: 289535