[clang] -fsanitize=alignment: check memcpy/memmove arguments (PR #67766)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Thanks for the comment.

> I think the choice we're making here is probably worth it, though we should probably document it better. I think you can remove the alignment assumption by explicitly casting the operands to char* before passing them to memcpy; if you can't, I'd be more worried that we're doing something problematic here. 

Yes. The correct implementation correctly drops the alignment check on the dst parameter for `memcpy((char *)a, b, sz);`

> Also, it'd seem like a good idea to make the sanitizer message as clear as possible for this case, because Clang's behavior here is surprising.

@zygoloid 
Is reusing the message for regular stores clear (current behavior) enough?
```
// CHECK-MEMCPY-STORE: misaligned.cpp:[[#@LINE+4]]{{(:12)?}}: runtime error: store to misaligned address [[PTR:0x[0-9a-f]*]] for type 'int *', which requires 4 byte alignment
```


https://github.com/llvm/llvm-project/pull/67766