[SPIRV] Add the `spirv.Layout` type to the SPIR-V backend.

[s-perron]

Add this type in accordance with the design doc.

TL/DR; This type is needed because CBuffers have a feature where the developer can set the offset of a particular member, and there are no restrictions on that member. We also want to keep the order of the member in llvm-ir the same as the declaration order in HLSL. This means we cannot use the standard algorithm to determine the offset of a member in a CBuffer struct.

[s-perron]

Waiting for #135789 to merge.