[clang][Sema] Add truncation warning on fortified snprintf

This patch warns on snprintf calls whose n argument is known to be smaller than the size of the formatted string like

```
char buf[5];
snprintf(buf, 5, "Hello");
```
This is a counterpart of gcc's Wformat-truncation
Fixes https://github.com/llvm/llvm-project/issues/64871

Reviewed By: aaron.ballman, nickdesaulniers
Differential Revision: https://reviews.llvm.org/D158562