Fix lldb-vscode builds on Windows targeting POSIX

@stella.stamenova found out that lldb-vscode's Win32 macros were failing
when building on windows targetings POSIX platforms.

I'm changing these macros for LLVM_ON_UNIX, which should be more
accurate.

GitOrigin-RevId: 0bca9a7ce2eeaa9f1d732ffbc17769560a2b236e
6 files changed