spirv-to-nir opencl fixes
Here's implementations of a bunch of SPIR-V OpenCL-specific opcodes. While some of these have implementations libclc for the corresponding functionality, some do not, and even the ones that have can often be used by using the native_
-variations of CL functions, or simply by not compiling the kernels using OpenCL C as the source language. We can also build Clover without libclc support.