[OpenCL] Fix file-scope const sampler variable for 2.0
OpenCL spec v2.0 s6.13.14:
Samplers can also be declared as global constants in the program
source using the following syntax.
const sampler_t <sampler name> = <value>
This works fine for OpenCL 1.2 but fails for 2.0, because clang duduces
address space of file-scope const sampler variable to be in global address
space whereas spec v2.0 s6.9.b forbids file-scope sampler variable to be
in global address space.
The fix is not to deduce address space for file-scope sampler variables.
Differential Revision: https://reviews.llvm.org/D62197