[llvm] [BOLT][AArch64] Preserve external branches to non-function symbols (PR #227275)

[Adam Bzowski via llvm-commits]

adam-bzowski-arm wrote:

I think this is a good idea but it is incomplete yet. I currently get a crash (full log attached, [brbe.google.br.log](https://github.com/user-attachments/files/32803974/brbe.google.br.log)):

```text
llvm-bolt: LongJmp.cpp:1704: ClusteredRelaxation::relaxUnconditionalBranches():
Assertion `Target && "expected branch thunk chain"' failed.
```

One problem exposed by this change is that genuine compiler-generated tail calls are sometimes classified as external branches. For example, the destination has a local `STT_FUNC` symbol, but the retained relocation references the `STT_SECTION` `.text` symbol plus an addend. The linker likely canonicalized the relocation this way because the function symbol is local. Consequently, inspecting only the relocation symbol type does not recognize this as a function entry.

With my full-layout prototype, https://github.com/llvm/llvm-project/pull/225728, this case terminates with a diagnostic rather than an assertion failure. I will investigate further.

https://github.com/llvm/llvm-project/pull/227275