[llvm] [LLVM][TargetParser] Handle -msys targets the same as -cygwin. (PR #136817)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Notably, the msys triple is generated by the config.guess in this repo via llvm/modules/GetHostTriple.cmake:
> 
> https://github.com/llvm/llvm-project/blob/da8f2d52423bb82b5d4e75cff3018704effe044f/llvm/cmake/config.guess#L809-L811

Oh, good point. (And that bit is also available in upstream `config.guess` - the one we have here is an old copy of the upstream, with local modifications on top at this point.)

https://github.com/llvm/llvm-project/pull/136817