[clang] [llvm] [win][x64] Add support for Windows x64 unwind v3 (PR #200249)

[via cfe-commits]

Andarwinux wrote:

> > > > * Add an unwind v3 pass to chain unwind info if there are too many epilogs in a function and raise a fatal error if there are too many instructions in a prolog or epilog.
> > > 
> > > 
> > > Is this expected limitation of unwindv3, or is it some workaround?
> > > I encountered this problem when trying unwindv3 in an actual codebase:
> > > ```
> > > ld.lld: error: <unknown>:0: value of 308819 is too large for field of 2 bytes
> > > 
> > > 
> > > ld.lld: error: undefined symbol: typeinfo for MediaInfoLib::File_Adm
> > > >>> referenced by mediainfo.exe.lto.obj:(vtable for MediaInfoLib::File_Adm)
> > > ```
> > 
> > 
> > This is a fundamental limitation on unwind v3 - the encoding limits only provides so many bits to store counts and offsets.
> > Do you happen to have a reliable repro for that?
> > That error happens to be a generic "too big for a fixup" error:
> > https://github.com/llvm/llvm-project/blob/652915c8bb58dc5e8370fe417f64949207c0f593/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp#L710-L717
> > 
> > We need to know which of the various offsets is too large. Given the size of that, I wonder if it is because we're using the offset from the beginning of the function to the first epilog and should switch to using the offset from the end of the function to the last instead.
> 
> https://github.com/Andarwinux/_/releases/download/unwindv3/test.zip
> 
> This is a lld reproduce, you can run `lld-link @response.txt` to reproduce it.

I found a way to get a reduced IR reproducer.

[reduced.ll.txt](https://github.com/user-attachments/files/28805989/reduced.ll.txt)

```
clang -O3 -march=tigerlake --target=x86_64-pc-windows-gnu -c reduced.ll
<unknown>:0: error: value of 65699 is too large for field of 2 bytes
error: cannot compile inline asm
1 error generated.
```


https://github.com/llvm/llvm-project/pull/200249