[clang] [clang][ptrauth] Warn about the use of a weak signing schema (PR #157779)

[Martin Balao Alonso via cfe-commits]

martinuy wrote:

> A side thought: what is the expected effect of this warning (namely, treating implicit signing the same way as `__ptrauth` qualifiers) when building an existing code base for a new target that has function pointer signing enabled? My initial assumption is that in a ptrauth-unaware C code most function pointer uses would be diagnosed. I'm not sure what the effect **should** be, but I guess if my assumption is correct this could be quite unintended.
> 

It's a fair point, thanks for bringing it up. My thinking is that building a pointer authentication unaware code base for a new target that supports the feature shouldn't be enough reason for this warning to pop up. I would expect this warning only if a compilation argument that turns pointer authentication on is explicitly passed, such as `-fptrauth-calls`. If pointer authentication is intended to be on and the argument is, thus, passed, the warning would be a good opportunity to make a conscious decision: turn pointer authentication off, harden the code base, turn the warning off, etc.


https://github.com/llvm/llvm-project/pull/157779