[clang] [clang][Sema] Add buffer-size checks for unistd I/O functions (PR #224979)

[Venkatesh Srinivasan via cfe-commits]

venk-ks wrote:

Quick heads-up: #204337 (`fread`/`fwrite`/`fgets`) just landed on `main` and touched `checkFortifiedBuiltinMemoryFunction` in two ways that affect this PR on rebase:

1. `SourceSize` and `DestinationSize` in `checkFortifiedBuiltinMemoryFunction` were renamed to `AccessSize` and `BufferSize`.
2. It added `diag::warn_fortify_source_overread` (`InGroup<FortifySource>`, used by `fwrite`). Instead of calling `Checker.checkSourceOverread(1, 2)` (which emits `-Wstringop-overread`), `write`, `pwrite`, and `pwrite64` can now set:
   ```cpp
   DiagID = diag::warn_fortify_source_overread;
   AccessSize = Checker.ComputeExplicitObjectSizeArgument(2);
   BufferSize = Checker.ComputeSizeArgument(1);
   break;
   ```
   so that their overread warnings are emitted under `-Wfortify-source` as well (along with updating the `write`/`pwrite`/`pwrite64` warning expectations in the tests and `ReleaseNotes.md`).

https://github.com/llvm/llvm-project/pull/224979