[clang] [CIR] Support Aggregate Atomic to non atomic with padding (PR #219680)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Leaves quite a few NYIs in place that its a bit sad we aren't solving yet, but hopefully is enough to get the build moving.

THIS should be the inverse case? : 
```
struct T { char a, b, c; };   // size 3 => padded atomic representation
struct T g;
_Atomic (struct T) load(void) { return g; }
```

THIS is the peephole:
```
struct T { char a, b, c; };   // size 3 => padded atomic representation
_Atomic(struct T) g;
_Atomic (struct T) load(void) { return g; }
```

https://github.com/llvm/llvm-project/pull/219680