[HLSL][SPIR-V] Fix backend layout calculation

[Keenuts]

PushConstants require a specific layout rule. The previous way to emit this layout information was to emit a target-specific type with the field offsets as operands. This construct is being phased out in favor of explicit padding in the struct definition (see #147352)

This means the current PushConstant implementation will emit wrong offsets for some structs, and we should
re-visit this once we figure out the E2E path to correctly handle this layout information.

[s-perron]

This problem is that the SPIR-V backend's offset calculations do not seem correct.

@Keenuts Can you provide the code for the but you see?

One case I am seeing an error is:

%__cblayout_MyCBuffer = type <{ <{ [2 x <{ <3 x float>, target("spirv.Padding", 4) }>], <3 x float> }> }>

The SPIR-V backend is placing the padding at offset 16 in the struct. I would expect it to be at offset 12, immediately after the vector of 3 floats. Note that the "<>" around the struct types means that they should be packed, but we somehow end up with implicit padding.

It is hard to say if this is a FE problem (the implicit padding is expected, so the explicit padding should not have been added) or a backend problem.