Check if a terminal supports colors on Windows properly

Summary:
Previously we SetUseColor(true) wrongly when output was not a terminal so it broken some (not public) bots.

Thanks for issue report, @stella.stamenova

Reviewers: stella.stamenova, zturner

Reviewed By: stella.stamenova

Subscribers: abidh, lldb-commits, stella.stamenova

Differential Revision: https://reviews.llvm.org/D51772

llvm-svn: 341746