[lldb-vscode] correctly use Windows macros

@mstorsjo found a mistake that I made when trying to fix some Windows
compilation errors encountered by @stella.stamenova.

I was incorrectly using the LLVM_ON_UNIX macro. In any case, proper use
of

  #if defined(_WIN32)

should be the actual fix.

Differential Revision: https://reviews.llvm.org/D96060

GitOrigin-RevId: 36496cc2992d6fa26e6024971efcfc7d15f69888
6 files changed