ConsoleReporter sends INFO to stdout

[som-snytt]

Fixes #16701

Tested only manually.

For example, with tailrec printer enabled,

scalac -Vprint:typer example.scala > x 2> y

puts tree output and tailrec trace in x, warnings & errors & summary count in y.

x is the output I asked for. This makes it easier to see trace and trees correctly interleaved, since > out 2>&1 does not guarantee it.

Tests may depend on how output is captured. Scala 2 partest captures stderr to log, for example.

[som-snytt]

The "42 warnings found" message was INFO (undesired) and now WARN, but override in test reporter as no-op, as it is not needed for testing. (There must be 42 "warn" annotations. We assume the reporter knows how to count.) (On scala 2, the summary message is in the check file.)

In scala 2, ConsoleReporter#finish outputs the count. IIRC finish was introduced for just this purpose, a reporter-specific "footer" message. Here, doReport is used by Reporter#printSummary as a direct channel, since this message is not itself a countable warning. By contrast, summarizeUnreportedWarnings is a countable warning.