[clang] [clang-cl] Accept `cl`-style output arguments (`/Fo`, `-Fo`) for `--precompile` (PR #121046)

[Hans Wennborg via cfe-commits]

zmodem wrote:

> @zmodem given how important this it, I hope we can make this for clang21. Thanks!

I'm just a reviewer here, and I still find the patch very confusing.

And taking a step back, the whole point of clang-cl is to provide a cl.exe compatible interface to clang. I presume cl.exe does not use `--precompile -x c++-module` to produce BMI files. Shouldn't we try to match what they're doing?

https://github.com/llvm/llvm-project/pull/121046