[OpenCL] Implement as_type operator as alias of __builtin_astype.

Reviewers: Anastasia

Reviewed By: Anastasia

Subscribers: cfe-commits, yaxunl, bader

Differential Revision: https://reviews.llvm.org/D28136

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297947 91177308-0d34-0410-b5e6-96231b3b80d8