[OpenCL] Fixes bug of missing OCL version metadata on the AMDGCN target

Added the opencl.ocl.version metadata to be emitted with amdgcn. Created a static function emitOCLVerMD which is shared between triple spir and target amdgcn.

Also added new testcases to existing test file, spir_version.cl inside test/CodeGenOpenCL.

Patch by Aaron En Ye Shi.

Differential Revision: https://reviews.llvm.org/D22424

llvm-svn: 276010