Add a spirv-opt pass to add NonWritable to top-level structs when all members are NonWritable

A quirk of DXC that went unnoticed for years is that instead of decorating the top-level of a buffer as NonWritable, it decorates all members of the buffer. While this is legal, it wasn't noticed during bring up of VK_EXT_descriptor_heap, and a key part of how the mapping APIs in that extension works is making use of this decoration.

A [DXC issue has been raised](https://github.com/microsoft/DirectXShaderCompiler/issues/8492) to modify DXC to adjust for this, but for existing shaders it could be useful to be able to do a quick fixup pass, without needing to regenerate all of the shaders.

This issue is a request to at least consider adding a pass to do this to spirv-opt.

Start by reviewing the spirv-opt pass framework and existing decoration-related passes. Determine how to identify top-level buffer structs whose members are all NonWritable, then verify that the pass produces the requested top-level decoration without changing unrelated shaders.

Written by the indexing model from the issue text.