[raw_ostream] When printing color on Windows, use correct bg color.

When using SetConsoleTextAttribute() to set the foreground or
background color, if you don't explicitly set both colors, then
a default value of black will be chosen for whichever you don't
specify a value for.

This is annoying when you have a non default console background
color, for example, and you try to set the foreground color.

This patch gets the existing fg/bg color and when you set one
attribute, sets the opposite attribute to its existing color
prior to comitting the update.

Reviewed by: Aaron Ballman
Differential Revision: http://reviews.llvm.org/D7967

llvm-svn: 230859