[llvm] [PtrAuth] Add ConstantPtrAuth comparator to FunctionComparator.cpp (PR #159480)

[Anatoly Trosinenko via llvm-commits]

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atrosinenko wrote:

Have you considered auto-generating the CHECK lines with `llvm/utils/update_test_checks.py`?

[nit] It may be better to pass the input file by [redirecting it to stdin](https://llvm.org/docs/TestingGuide.html#fragile-tests):
```
; RUN: opt -passes=mergefunc -S < %s | FileCheck %s
```

https://github.com/llvm/llvm-project/pull/159480