SI-9350 Command option -Xreporter

Add a setting to take a custom Reporter.

Example of reporter that discounts deprecations for purposes of
(not) failing the build:

```
import scala.tools.nsc.Settings
import scala.tools.nsc.reporters.ConsoleReporter

import scala.reflect.internal.util._

class MyReporter(ss: Settings) extends ConsoleReporter(ss) {
  var deprecationCount = 0
  override def warning(pos: Position, msg: String): Unit = {
    if (msg contains "is deprecated") deprecationCount += 1
    super.warning(pos, msg)
  }
  override def hasWarnings: Boolean = count(WARNING) - deprecationCount > 0
  override def reset() = { deprecationCount = 0 ; super.reset() }
}
```

Invoked as:

```
$ scalac -toolcp . -Xreporter myrep.MyReporter -Xfatal-warnings -deprecation test.scala
test.scala:8: warning: class C in package tester is deprecated: Don't use me
  Console println s"${new C}"
                          ^
one warning found
```

where the reporter class is in the current directory, placed on the tool class path.

Also flush on early-reported errors.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala b/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
index c9d97a9469..c8251a542c 100644
--- a/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
+++ b/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
@@ -134,6 +134,7 @@ trait ScalaSettings extends AbsScalaSettings
   val Xshowobj           = StringSetting       ("-Xshow-object", "object", "Show internal representation of object.", "")
   val showPhases         = BooleanSetting      ("-Xshow-phases", "Print a synopsis of compiler phases.")
   val sourceReader       = StringSetting       ("-Xsource-reader", "classname", "Specify a custom method for reading source files.", "")
+  val reporter           = StringSetting       ("-Xreporter", "classname", "Specify a custom reporter for compiler messages.", "scala.tools.nsc.reporters.ConsoleReporter")
   val strictInference    = BooleanSetting      ("-Xstrict-inference", "Don't infer known-unsound types")
   val source             = ScalaVersionSetting ("-Xsource", "version", "Treat compiler input as Scala source for the specified version, see SI-8126.", initial = ScalaVersion("2.11"))