Set console mode when -fansi-escape-codes is enabled

Summary:
Windows console now supports supports ANSI escape codes, but we need to enable it using SetConsoleMode with ENABLE_VIRTUAL_TERMINAL_PROCESSING flag.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38817


Tested on Windows 10, screenshot:
https://i.imgur.com/bqYq0Uy.png

Reviewers: zturner, chandlerc

Reviewed By: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51611

llvm-svn: 341396