Allow [[vk::location()]] on a global variable

[Keenuts]

Describe the solution you'd like

Current inline SPIR-V allows:

• use vk::ext_decorate_id on a global variable.
• use vk::ext_storage_class on a global variable.

This means we can declare a variable with the Location decoration, and the Input storage class.
Seems to me we should also allow adding vk::location on such globals, as we can already attach a Location to a global, but the ext_decorate_id path is less readable.

Describe alternatives you've considered
Keep as-is, and force users to use the less readable vk::ext_decorate_id version.

[Keenuts]

@s-perron I think this straightforward