[clang] Add check in SemaChecking for multiple unsequenced volatile accesses (PR #180955)

[Aaron Ballman via cfe-commits]

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AaronBallman wrote:

I'd also like to see some additional tests like:
```
volatile int v1, v2;

x = v1 + v2; // Okay, accessing different values

volatile int *pv1 = v1, *pv2 = v1; // Pointing to same volatile object
x = *pv1 + *pv2; // unsequenced, but might be a FIXME if we're not tracking pointer ownership
```

https://github.com/llvm/llvm-project/pull/180955