[mlir][spirv] Add utilities for push constant value

This commit add utility functions for creating push constant
storage variable and loading values from it.

Along the way, performs some clean up:

* Deleted `setABIAttrs`, which is just a 4-liner function
  with one user.
* Moved `SPIRVConverstionTarget` into `mlir` namespace,
  to be consistent with `SPIRVTypeConverter` and
  `LLVMConversionTarget`.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D99725