[HLSL][SPIRV] Guess at the image format for storage images

[s-perron]

Even in VK1.3, not all driver have to support reading and writing to a storage image without a format. If we were to use the unknown format, users may see a regression when compared to DXC. To make things easier to port, we will implement microsoft/DirectXShaderCompiler#7632. At some point we may decide to make it the default.

• Add the new option -fspv-use-unknown-image-format to clang-dxc.
• Update the wg-hlsl spir-v resource proposal to reflect this change.