[HLSL][SPIR-V] Add RWStructutredBuffer type support for SPIR-V

[s-perron]

The HLSL structured buffers are an HLSL type that need to be supported in the backend.

The design for implementing this resource is in llvm/wg-hlsl#98.

The tasks to accomplish are:

• Add the desired target type to the SPIR-V backend. We want to use spirv.Type to represent the resource.
• Update clang to use the correct target type, with the block decoration.
• Update the expansion of @llvm.spv.resource.getpointer to handle storage buffers. They have one less level of indirection than a storage texel buffer.
• Resubmit the tests removed in [SPIRV] Expand RWBuffer load and store from HLSL [SPIRV] Expand RWBuffer load and store from HLSL #122355 that relate to textures. We need the intrinsics to access the textures to enble thoses tests.

[EugeneZelenko]

@s-perron: Please don't forget to add relevant label(s) when creating issues.