[clang] [PAC] Implement function pointer re-signing (PR #98847)

[Daniil Kovalev via cfe-commits]

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+// RUN: %clang_cc1 %s -triple arm64e-apple-ios13 -fptrauth-calls -fptrauth-intrinsics -emit-llvm -o-  -fptrauth-function-pointer-type-discrimination | FileCheck %s
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kovdan01 wrote:

Nit: it would also be nice to have RUN lines against linux triples. I'm OK with adding that in a subsequent PR separately (probably by myself :) ). For example:

```
// RUN: %clang_cc1 %s -triple aarch64-linux-gnu  -fptrauth-calls -fptrauth-intrinsics -emit-llvm -o-  -fptrauth-function-pointer-type-discrimination | FileCheck %s
```

It'll require a slight change in CHECK-LABEL lines (for linux triple, we'll have dso_local). I'm happy to do such test enhancement by myself later, so feel free to ignore.

https://github.com/llvm/llvm-project/pull/98847