[spirv-val] The atomic 'storage class forbidden by universal validation rules' error should print out the forbidden storage class.

I'm getting
```
AtomicExchange: storage class forbidden by universal validation rules.
```
when running `spirv-val` on a file. This error is very vague as it prints out the opcode that's doing something illegal, but not the forbidden storage class.

Ideally we want something like

```
Opcode AtomicExchange: storage class 'Output' forbidden by universal validation rules.
```

This should be any easy issue for anyone to pick up. The code in question is here: https://github.com/KhronosGroup/SPIRV-Tools/blob/4b092d2ab81854e61632bdd1e658907f0071c37e/source/val/validate_atomics.cpp#L194-L199.

Inspect source/val/validate_atomics.cpp at lines 194-199, where the atomic validation diagnostic is assembled. Update the message so it includes the forbidden storage class, then run spirv-val on the reported input and verify that the output names the storage class as shown.

Written by the indexing model from the issue text.