[naga spv-in] Support atomic operations on fields of global structs

[schell]

Currently when an atomic operation is performed on a field in a global struct, the SPIR-V frontend's atomic upgrade machinery upgrades all of the fields in the struct errs with ``.

It should upgrade successfully without performing type upgrades on all fields.
In other words, upgrades should be done only for the one field that gets used atomically.

Part of

• [spv-in] Atomics support #4489

[schell]

Fixed with #6693