[clang] [lldb] [llvm] [Support][Driver][LLDB] Guard WIN32_LEAN_AND_MEAN definitions with #ifndef (PR #213045)

[Saleem Abdulrasool via cfe-commits]

https://github.com/compnerd approved this pull request.

I suppose that this is okay. I would rather that we hoist this into CMake and remove it from the headers.

Why the indentation changes?

https://github.com/llvm/llvm-project/pull/213045