Split the Windows toolchain definition into its own file. This is

especially nice as the Windows toolchain needs the windows header files,
and has lots of platform specific hooks in it.

To facilitate the split, hoist a bunch of file-level static helpers into
class-level static helpers. Spiff up their doxygen comments while there
as they're now more likely to be looked up via docs.

Hopefully, this will be followed by further breaking apart of the
toolchain definitions. Most of the large and complex ones should likely
live on their own. I'm looking at you Darwin. ;]

llvm-svn: 146840