Logging: Add log stream that only get sent warning+ logs.

[Logikable]

We'd like to subscribe to only the most critical logs. Currently, there is no way to specify a log level threshold. This PR provides a stopgap solution.

In addition to the log_streams vector that new log sinks can push to, this PR adds a log_warning_streams vector that behaves similarly, but ostreams in this new vector only receive warning and error logs.

[mikesinouye]

Hey @widlarizer, please take a look when you get a chance, thank you!

[widlarizer]

Seems like ERROR is predefined somewhere in Windows specific headers. You can rename the enum variand or #ifdef #undef it

[Logikable]

Renamed, thanks :)

[widlarizer]

I'm in favor of the LogSeverity struct and an overloaded log function seems like a decent way to safely retrofit it. Is there an intended followup that exposes log_warning_streams somehow? As a side note, please take care to respect established whitespace, C++ is indented with tabs

[Logikable]

I've rewritten this PR in light of the log changes by @rocallahan. One notable consequence of this PR is that log_formatted_warning and log_error_with_prefix now call log_formatted_string instead of log. I've also switched to tabs.

Regarding the last comment (@widlarizer): log_warning_streams is already exposed the same way log_streams is exposed. Today, to subscribe a sink, you append an ostream to the global Yosys::log_streams; the same can be done with log_warning_streams.

[widlarizer]

There is no code in this PR appending to log_warning_streams

[widlarizer]

Also, this breaks tests/various/logger_cmd_error.sh

[Logikable]

Sorry -- I misunderstood. We'll be appending to log_warning_streams in a downstream change. I felt it better to contribute the public parts of our change upstream; is that ok?