Hide stack traces behind -Ydebug

[smarter]

They're not very useful for end users and some tests like
tests/neg/selfreq.scala always print these exceptions which makes it
harder to read the test logs,

Also use Thread.dumpStack() instead of creating an Exception and calling
printStackTrace() on it.

Review by @odersky or @DarkDimius

[smarter]

Stack traces are useful documentation when something goes wrong. Users should paste them into their bugreports. Turning them off by default makes it harder to troubeshoot.

Look what happens when a user tries to compile tests/neg/selfreq.scala:
https://gist.github.com/anonymous/0f49ed5beb84d51f097a
200 lines of mostly useless output, the user will probably think that there is a bug in the compiler, even though this is just debug information.

I'm against disabling stacktraces in partest. They provide valuable information in case of failure.

Note that I'm not disabling stacktraces when the compiler fails with an exception, I'm only disabling stack traces that are explicitly printed in random places in the compiler with printStackTrace() or dumpStack(). In master currently we have 15 cases where this happen, 4 of these cases are hidden behind if (ctx.debug), why these 4? Don't you think that we should be consistent?

[odersky]

OK, I understand the reasoning that we want to hide these stacktraces by default because they do not signify real compiler crashes.

Can you try to do the whole test suite under -Ydebug? If that passes, we are good. If not, we need to either debug -Ydebug first, or else create a more specific option (-Ydebug:stacktraces?), or else
use a printer in Printers.scala for it. In fact it strikes me that we could generalize the cyclicErrors printer to all stack traces.

[smarter]

Can you try to do the whole test suite under -Ydebug? If that passes, we are good

This now works after dotty-staging@a6719ec and dotty-staging@ee67cd9

[DarkDimius]

@smarter could you maintain dotty-staging@a6719ec under a -Ytrace-context-creation? It proved very useful while debugging typer&tree transforms.

[smarter]

@DarkDimius : done!

[smarter]

/rebuild