Header file to help forcibly link GPURuntime

Summary: LinkGPURuntime.h defines and creates a structure ForceGPURuntimeLinking which creates an artificial dependency to functions defined in GPUJIT.c. The presence of this structure ensures that these functions are a part of the compiled object/library files including it.

Reviewers: grosser, Meinersbur

Reviewed By: grosser

Subscribers: #polly, pollydev

Tags: #polly

Differential Revision: https://reviews.llvm.org/D33198