[clang] [analyzer] Make recognition of hardened __FOO_chk functions explicit (PR #86536)

[via cfe-commits]

NagyDonat wrote:

[Replying to the following inline comment of @steakhal :]
> You had a comment about __builtin___vsprintf_chk.
```
  // This is using `equals()` instead of more lenient prefix/suffix/substring
  // checks because we don't want to say that e.g. `__builtin___vsprintf_chk()`
  // is a hardened variant of `sprintf()`.
  ```
> I wonder if we could have a test demonstrating that.

Instead of demonstrating this with `sprintf` / `vsprintf`, I ended up creating testcases that use `memcpy` / `wmemcpy`, because in the non-hardened case the analogous problem was tested with `memcpy` / `wmemcpy`. I wrote this comment before creating those testcases; now that the testcases are there, I removed the comment (by merging your suggestion that tweaks the adjacent code).

https://github.com/llvm/llvm-project/pull/86536