[clang] Patch series to reapply #118734 and substantially improve it (PR #120534)

[Chandler Carruth via cfe-commits]

chandlerc wrote:

> > > Just to add some more details now that I've slept a bit...
> > > Previously there were errors in AArch64 and RISCV -- it'll be really useful to know if those are the only errors with this patch, are there new ones, and especially if the RISCV errors go away that might be helpful. If the errors are _only_ in AArch64, then I have ideas to further improve this.
> > > Also, last time that @zmodem helped look at the `.obj` file for this, I think we ended up looking at the wrong one -- I don't think it is the `clang/lib/Basic/Builtins.obj` that would show signs of the issue, but `clang/lib/Basic/Targets/AArch64.obj` (and potentially the `RISCV.obj`). Either those `.obj` files should be clearly wrong (IE, some of the arrays truncated / zeroed /etc), or this is a mis-_link_ not a mis-_compile_...
> > > But my goal is to somewhat experimentally find the threshold below which things start working. With this patch series there is an architecture to mechanically cut things down more and more if needed, but need some idea of the target I should aim for.
> > 
> > 
> > Sure. I deleted the object and log files, but can easily run it again. IIRC the tests that I saw failing in the console were LoongArch, but I think many/most targets were affected.
> > I'll take the buildbot offline, run it again and then archive the logs and object files somewhere.
> 
> Here is the logfile from the run. This is doing the exact same run as the buildbot (I ran the same commands as the buildbot would run). The compressed logfile is around 15MB, the uncompressed is around 300MB.
> 
> [dropbox.com/scl/fi/urcanfwpf6da1dz2i5t85/log_test.zip?rlkey=5ludb4vpoe32mfp7hlnelrr47&dl=0](https://www.dropbox.com/scl/fi/urcanfwpf6da1dz2i5t85/log_test.zip?rlkey=5ludb4vpoe32mfp7hlnelrr47&dl=0)
> 
> It's going to take a while to extract the build directory as it is 8GB in size, but if you have specific files you want to see let me know and I can pick them out.
> 
> Note that our bot doesn't build ARM/AArch64, so it would make sense that we don't see failures there (although I can't remember if we did previously...)

Thanks, this is already really helpful.

The bots do seem to build the Clang parts of AArch64:

```
PASS: Clang :: AST/builtins-arm-strex-rettype.c (438 of 82164)
```

This was one the failures previously, and is now passing. I'm not seeing any of the AArch64, ARM, or RISCV failures that I was seeing. But there are still x86 and LoongArch failures. That may point me at how to make progress.

Let me work on adding a few more patches to the series.

https://github.com/llvm/llvm-project/pull/120534