[OpenCL] Fix code generation of function-scope constant samplers.

Summary:
Constant samplers are handled as static variables and clang's code generation
library, which leads to llvm::unreachable. We bypass emitting sampler variable
as static since it's translated to a function call later.

Reviewers: yaxunl, Anastasia

Reviewed By: yaxunl, Anastasia

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34342

llvm-svn: 318290