Storage class for cooperative matrix.

[tvvikram]

The cooperative matrix spec states the following:
a. Under overview, we have: "cooperative matrix" types, where the storage for and computations performed on the matrix are spread across a set of invocations such as a subgroup.
b. Under validation rules, we have: Cooperative matrix types (or types containing them) can only be allocated in Function or Private storage classes.

It is a bit confusing as the Function / Private storage classes are not spread across a set of invocations.

A request for clarifying this was raised in https://gitlab.khronos.org/spirv/SPIR-V/-/issues/824 and it was suggested there that this can be a topic for SPIRV-Guide.

Clarification: The idea here is coopMat types in the program are referred to be in Function / Private storage whereas the underlying type that supports coopMat type can be in a storage other than Function / Private like Workgroup storage class. It also depends on multiple mental models that work out for coopMats.