[DX] Add support for PSV signature elements

The pipeline state data captured in the PSV0 section of the DXContainer
file encodes signature elements which are read by the runtime to map
inputs and outputs from the GPU program.

This change adds support for generating and parsing signature elements
with testing driven through the ObjectYAML tooling.

Reviewed By: bogner

Differential Revision: https://reviews.llvm.org/D157671