[clang] [clang][Sema] Add fortify warnings for `unistd.h` (PR #161737)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

I don't think that this is the best approach, it requires a lot of duplicated checks, and those checks are significantly removed from the primary bound check.

I think a better approach would be to instead change this code to record:

* The true size of the destination object (i.e. ignore the explicitly stated size)
* The true size of the source object (ditto)
* The minimum requested operation size (e.g. size parameter in memcpy)
* The maximum operation size (size in strl/strncpy)

Then have a single set of tests at the end that verify the minimum size is <= to the source and destination buffers, and the destination size vs the stated maximum operation size, and when relevant the object sizes (e.g. `char foo[5]; strcpy(foo, "too long");`).

Independently of all of this it might be nice to warn on `strncpy(buffer[5], "seven", 5)` due to the truncation of the null termination - again completely unrelated to this PR.


https://github.com/llvm/llvm-project/pull/161737