Removed redundant global.params.os field.

I hope I have untangled the checks for "native" Windows (Triple::Win32)
vs. Windows/MinGW/Cygwin (Triple::isOSWindows) correctly.

MinGW needs some default libraries as well, has to be fixed later.
Small cleanup.
Consistently use quotes for LLVM includes.
Sort includes according to style guidelines:

 1. Main include corresponding to .cpp file, if any.
 2. DMD and LDC includes.
 3. LLVM includes.
 4. System includes.

Also updated a few include guards to match the default format.