[clang] [clang] Function type attribute to prevent CFI instrumentation (PR #135836)

[via cfe-commits]

PiJoules wrote:

I updated to this with a new AST node but would like some guidance in implementation details. One of the shortcomings of making this a (sugared) node is when passing this type around through templates, the node doesn't persist since only canonical types are substituted through templates. This could lead to something like:

```
void func() __attribute__((cfi_unchecked_callee)) {}
template <typename T>
struct S {
  S(T *ptr) {}
};
S s(&func);
```

Giving a warning since clang deduces `T *ptr` as the canonical `void (*)()`. Other attributes like `noreturn` work around this by being part of the function prototype via `ExtInfo` or `FunctionTypeExtraBitfields` as @efriedma-quic suggested, but using the bitfields alone wouldn't allow warning on casts since those attrributes get freely added/dropped without warning. I think perhaps the best way to approach this would be instead treating this as an extra bitfield rather than having a node and for every implicit conversion which would add or drop attributes, specifically check if this attribute is dropped. I'd like some opinions on this before moving forward though just to make sure I don't do unnecessary work. I imagine we wouldn't want a mix of both a node and extra bitfield since we'd have two representations of the same thing. Another idea is perhaps making this not a sugared type, but I don't know if there would be ramifications to that.

https://github.com/llvm/llvm-project/pull/135836