[clang] [-Wunsafe-buffer-usage] Warning Libc functions (PR #101583)

[Mikael Holmén via cfe-commits]

mikaelholmen wrote:

We're seeing a crash with this patch when compiling with -Weverything.
```
clang: ../../clang/include/clang/AST/Expr.h:3026: const clang::Expr *clang::CallExpr::getArg(unsigned int) const: Assertion `Arg < getNumArgs() && "Arg access out of range!"' failed.
```
I'm working on extracting a reproducer.

https://github.com/llvm/llvm-project/pull/101583