Log to stderr as well as the log file, on non-Darwin platforms

[Wilfred]

This is consistent with other LSP servers, and makes debugging sourcekit-lsp easier.

This was initially discussed in #2116. Let me know if you think we should log to stderr on macOS too.

[ahoppen]

We do log to stderr until we set up the global log file handlers using setUpGlobalLogFileHandler. We should also log to stderr from there. With the current implementation, we would log every message twice if setting up the global log file handlers failed (once from the log handler and then once again from your fputs call).

[Wilfred]

Gotcha, presumably something like this is better?

[bnbarham]

@swift-ci please test

[bnbarham]

@swift-ci please test

[bnbarham]

Let me know if you think we should log to stderr on macOS too.

I'm not sure this is actually possible with our current implementation. All the calls are direct to OSLog and NonDarwinLogger is just API compatible with it. We can't wrap it, as we'd then lose its performance optimizations and ability to set privacy levels.

That also makes me wonder if we want this in general, since it means we have very different output to LSP clients between the platforms.

EDIT: Ah, it seems like various editors also treat stderr output as actual errors (eg. neovim logs stderr output with the format "[ERROR][timestamp] ..."). So it may indeed be best to avoid doing this generally. I've opened #2161 which seems like a better direction to go here.