[llvm] r215303 - ADT: remove MinGW32 and Cygwin OSType enum

[Yonggang Luo]

I also suggest add the mingw32 mingw-w64 related comment at the end of GNU  
enum, for newbies to identifier where
is mingw related things
  enum EnvironmentType {
137	    UnknownEnvironment,
138	
139	    GNU,
140	    GNUEABI,