[clang] [llvm] [AArch64][PAC] Sign block addresses used in indirectbr. (PR #97647)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The issue is just that clang expects the following to compile.  And with the current version of the patch, I think we end up crashing in the backend.

```
@f.x = internal global i32 trunc (i64 sub (i64 ptrtoint (ptr blockaddress(@f, %A) to i64), i64 ptrtoint (ptr blockaddress(@f, %B) to i64)) to i32), align 4
```

https://github.com/llvm/llvm-project/pull/97647