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
6dd07fa513