Validate storage class OpenCL environment rules for atomics (#2750)

This change refactors all storage class validation for atomics
to reflect the similar refactoring in the specification.

It is currently not possible to write a test for the check
rejecting Generic in an OpenCL 1.2 environment as the required
GenericPointer capability isn't allowed there. I've decided
to keep the check nonetheless to guard against the capability
becoming available without the rules for atomics being updated.

The ID changes in existing tests aren't ideal but introducing
names drags in a substantial refactoring of this file.

Contributes to #2595.

Signed-off-by: Kevin Petit <kevin.petit@arm.com>