[lld] [lld] lld/ELF/Writer.cpp: write directly to outs when output is "-" (PR #72061)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Ah, thanks for the explanation. The description can mention `raw_ostream`:
> 
> > This lets the LLD ELF driver be used as a library with its output written to a `raw_ostream`.
> 
> `message` calls write to `stdoutOS`, so you'll need to set `disableOutput` if you use any feature that calls `message`.
> 
> I think this is still fine but we need a unittest in `lld/unittest/` (`ninja check-lld-unit`)

Adding the feature in https://github.com/llvm/llvm-project/pull/209064  with a unit test.

https://github.com/llvm/llvm-project/pull/72061