s/LLVM_ON_WIN32/_WIN32/, lldb

                                                                                
LLVM_ON_WIN32 is set exactly with MSVC and MinGW (but not Cygwin) in            
HandleLLVMOptions.cmake, which is where _WIN32 defined too.  Just use the        
default macro instead of a reinvented one.                                      
                                                                                
See thread "Replacing LLVM_ON_WIN32 with just _WIN32" on llvm-dev and cfe-dev.  
No intended behavior change.

llvm-svn: 329697