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

[Venkatesh Srinivasan via cfe-commits]

venk-ks wrote:

Thanks for updating the PR to use `Builtins.td` and `switch (BuiltinID)`. This looks much cleaner.

Two small suggestions based on @nickdesaulniers's recent review feedback on #223520 and #226261:

1. In `SemaChecking.cpp`, when `Prototype = ""` is unset in `Builtins.td`, @nickdesaulniers requested also checking the types of the remaining arguments at the call site (see https://github.com/llvm/llvm-project/pull/226261#discussion_r4107750442):
   - `arg 0` (`fd`) is `isIntegerType()` for `read`, `write`, `pread`, `pread64`, `pwrite`, `pwrite64`, and `readlinkat`
   - `arg 3` (`offset`) is `isIntegerType()` for `pread`, `pread64`, `pwrite`, and `pwrite64`
   - `path` (`arg 0` for `readlink`, `arg 1` for `readlinkat`) is `isPointerType()`

2. In `Builtins.td`, could you add a comment above `let Prototype = "";` on each definition showing the C signature and which types are target-specific (e.g., `// ssize_t(int, void*, size_t); ssize_t is target-specific`)? Also, `PRead`/`PRead64` and `PWrite`/`PWrite64` can optionally be combined using `let Spellings = ["pread", "pread64"];` and `let Spellings = ["pwrite", "pwrite64"];`.

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