NFC patch which moves `DiagnosticsRendering` from `Utility` to `Host`.
This refactoring is needed for
https://github.com/llvm/llvm-project/pull/168603. It adds a method to
check whether the current terminal supports Unicode or not. This will be
OS dependent and a better fit for `Host`. Since `Utility` cannot depend
on `Host`, `DiagnosticsRendering` must live in `Host` instead.
0c7d826129