[clang] [libcxx] [clang] Warn about memset/memcpy to NonTriviallyCopyable types (PR #111434)

[Nathan Chancellor via cfe-commits]

nathanchance wrote:

> @nathanchance could you share your preprocessed source so that I can get a reproducer? Thanks!

Sure thing!

```
$ clang --target=x86_64-linux-gnu -fcf-protection -fsyntax-only -Wno-{address-of-packed-member,constant-logical-operand,gnu,unused-value} -Wnontrivial-memaccess rings.i
rings.i:95832:214: warning: first argument in call to '__builtin_memset' is a pointer to non-trivially copyable type 'struct nfp_nfd3_tx_desc' [-Wnontrivial-memaccess]
 95832 |  ({ size_t __fortify_size = (size_t)(tx_ring->size); fortify_memset_chk(__fortify_size, __builtin_dynamic_object_size(tx_ring->txds, 0), __builtin_dynamic_object_size(tx_ring->txds, 1)), __builtin_memset(tx_ring->txds, 0, __fortify_size); });
       |                                                                                                                                                                                                                      ^
rings.i:95832:214: note: explicitly cast the pointer to silence this warning
 95832 |  ({ size_t __fortify_size = (size_t)(tx_ring->size); fortify_memset_chk(__fortify_size, __builtin_dynamic_object_size(tx_ring->txds, 0), __builtin_dynamic_object_size(tx_ring->txds, 1)), __builtin_memset(tx_ring->txds, 0, __fortify_size); });
       |                                                                                                                                                                                                                      ^
       |                                                                                                                                                                                                             (void*)
1 warning generated.
```

[rings.i.txt](https://github.com/user-attachments/files/17560453/rings.i.txt)

https://github.com/llvm/llvm-project/pull/111434