llvm dev - Sep 2021 - warning for overloads of builtins

I'd like to implement an attribute to allow Fortify diagnosing of new
functions as if they were builtins. I'd appreciage any feedback about the
design.

An example illustrating a warning I'd like to happen:
///////////
void* memset(void* const s __attribute__((pass_object_size(0))),
             int c,
             int n) __attribute__((overloadable)) {
  return __builtin_memset(s, c, n);
}

int main() {
  char s[10];
  memset(s, 0, 11);
}
//////////

Ideally Fortify would be able to diagnose the overflow in the call to this
overloaded memset. Apparently this used to happen, but it stopped working
after this patch (https://reviews.llvm.org/D77491).

The design I'm thinking would involve adding a new diagnose_as_if
attribute, so that we could indicate that function calls to one function
should be diagnosed as if they were calls to another. For instance, it
might look like this:

//////////void* mymemset(void* const s,
             int c,
             int n) __attribute__((diagnose_as_if(__builtin_memset(s, c,
n)))) {
  return __builtin_memset(s, c, n);
}
///////

I see several ways the diagnose_as_if attribute could work:

1. diagnose_as_if(__builtin_memset) (just diagnose `mymemset` as if it's a
call to __builtin_memset, with arguments in the same order)

2. diagnose_as_if(__builtin_memset(0, 1, 2)) (use integers to specify which
parameters of the call to `mymemset` go in which argument of
`__builtin_memset`)

3. diagnose_as_if(__builtin_memset(s, c, n) (use parameter names to specify
which parameters of the call to `mymemset` go in which argument of
`__builtin_memset`)

4. As an extension to item 3, potentially the arguments of
`__builtin_memset` could be more complicated expressions than just names of
parameters, like `__builtin_memset(s + 2, c, n)` or something. It seems
this would be more complicated to implement.

It's not immediately clear how much generality would be useful here; would
option 3 be sufficient? Or even option 1?
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