[Support] Expose flattenWindowsCommandLine.

This function was internal to Program.inc, but I've needed this
on several occasions when I've had to use CreateProcess without
llvm's sys::Execute functions.  In doing so, I noticed that the
function was written using unsafe C-string access and was pretty
hard to understand / make sense of, so I've also re-written the
functions to use more modern LLVM constructs.

llvm-svn: 334353