TSCBasic: always report a dumb terminal on Windows
TSCBasic: always report a dumb terminal on Windows

Although Windows Terminal should provide a proper tty, for now, always
report a dumb terminal.  This repairs the rendering of C/C++
diagnostics.

Thanks to @albertelrud for the amazing pointer on what it may be!

apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core