[polly-acc] change cl_get_* return types to 32/64bit

Summary:
This patch changes the return types for ocl_get_* functions during SPIR code generation. Because these functions return size_t types, the return type needs to be changed to the actual size of size_t on the device.

Based on work by Michal Babej and Pekka Jääskeläinen

Patch by: Alain Denzler

Reviewers: grosser, philip.pfaffe, bollu

Reviewed By: grosser, philip.pfaffe

Subscribers: nemanjai, kbarton, llvm-commits

Differential Revision: https://reviews.llvm.org/D48774

llvm-svn: 336080