spirv-val: doesn't seem to check for `VK_KHR_workgroup_memory_explicit_layout` extension and capability

Validation doesn't seem to check for `VK_KHR_workgroup_memory_explicit_layout` extension and capability when `Offset` decoration is added to a `groupshared` variable.

This passes validation: https://godbolt.org/z/crPdhdfPG
Expected: https://godbolt.org/z/jYcxsvx7a

Start by reproducing the validation difference using the two linked Godbolt examples and the spirv-val entry point. Trace validation for an Offset decoration on a groupshared variable, then confirm that the VK_KHR_workgroup_memory_explicit_layout extension and capability are enforced as shown by the expected example.

Written by the indexing model from the issue text.