[clang] [Clang][Sema][Builtins] Check argument count for `__builtin_allow_sanitize_check` (PR #183927)

[Thibault Monnier via cfe-commits]

================
@@ -3816,6 +3804,9 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
   }
 
   case Builtin::BI__builtin_allow_sanitize_check: {
+    if (checkArgCount(TheCall, 1))
----------------
Thibault-Monnier wrote:

I've been trying to work on this. Many builtins have variadic signatures: is there any way to make the arg count check also work on these?

I wrote a check that looks like this:
```cpp
if (FPT->isVariadic() ? checkArgCountAtLeast(TheCall, NumParams)
                      : checkArgCount(TheCall, NumParams))
```
However, this is messy because all builtins using a variadic signature must implement an additional check later on, and the variadic check straight up doesn't work in C mode (where variadic signatures are `FunctionNoProtoType` instead of `FunctionProtoType`).

To make things worse, the `_setjmp` builtin is usually defined as `int(jmp_buf)`, but in MinGW there is an extra parameter which breaks with this change. There's a `FIXME` about this issue (see [here](https://github.com/llvm/llvm-project/blob/a82e3a19258da70aecd0177a2c4b854a659b153f/clang/include/clang/Basic/Builtins.td?plain=1#L3442)).

https://github.com/llvm/llvm-project/pull/183927