[MLIR][SPIRVToLLVM] Added a hook for descriptor set / binding encoding

This patch introduces a hook to encode descriptor set
and binding number into `spv.globalVariable`'s symbolic name. This
allows to preserve this information, and at the same time legalize
the global variable for the conversion to LLVM dialect.

This is required for `mlir-spirv-cpu-runner` to convert kernel
arguments into LLVM.

Also, a couple of some nits added:
- removed unused comment
- changed to a capital letter in the comment

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D86515