SPIR-V: Validation rules for memory semantics storage classes?

Question from https://github.com/KhronosGroup/OpenCL-Docs/pull/89#issuecomment-503557202:

Memory Semantics is used to specify both memory-order constraints and the storage classes they apply to.

Make it explicit that only the memory-order constraint bits are restricted. The storage class can vary (WorkgroupMemory or CrossWorkgroupMemory).

do we want 1.2 validation rules for the storage classes? Can we define them more generally?

Start with the OpenCL-Docs pull request and comment linked in the issue, then review the surrounding SPIR-V memory-semantics validation rules. Clarify whether only memory-order constraint bits are restricted and whether storage-class rules should be defined generally; done means the specification records an agreed, explicit rule.

Written by the indexing model from the issue text.