[llvm] [COFF] Do not expose WinCOFFWriter to llvm namespace (PR #100505)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay commented:

Thanks for working on this, but this has potential limitation:

* `WinCOFFObjectWriter::WinCOFFWriter` is long and the nested type does not convey useful information 
* The size of `std::unique_ptr<WinCOFFWriter> ObjWriter, DwoWriter;` is still unknown, so users could not call `make_unique<WinCOFFObjectWriter>(...)` and remove `createWinCOFFObjectWriter`.

I think the ELFObjectWriter/ELFWriter relationship is the best.

https://github.com/llvm/llvm-project/pull/100505