SD-142 Avoid noisy log output in backend (#5134)

`withCurrentUnit` is designed to be called once per
compilation unit as it side effects by logging and updating
progress counters.

`GenBCode` was calling it more frequently (once per `ClassDef`.)
This is due to the somewhat convoluted internal architecture
of that phase, which is designed to support paralellism in
the future.

This commit factors out the internal part of `withCompilationUnit`
that modifies `currentUnit`, and calls that instead in the loop
over classes.

After this change:

```
% qscala -Ydebug
...
[running phase jvm on <console>] // only once
```

1 files changed, 8 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/Global.scala b/src/compiler/scala/tools/nsc/Global.scala
index 847c4cb2d1..94a6a0e4e2 100644
--- a/src/compiler/scala/tools/nsc/Global.scala
+++ b/src/compiler/scala/tools/nsc/Global.scala
@@ -394,15 +394,18 @@ class Global(var currentSettings: Settings, var reporter: Reporter)
 
       if (settings.debug && (settings.verbose || currentRun.size < 5))
         inform("[running phase " + name + " on " + unit + "]")
+      if (!cancelled(unit)) {
+        currentRun.informUnitStarting(this, unit)
+        try withCurrentUnitNoLog(unit)(task)
+        finally currentRun.advanceUnit()
+      }
+    }
 
+    final def withCurrentUnitNoLog(unit: CompilationUnit)(task: => Unit) {
       val unit0 = currentUnit
       try {
         currentRun.currentUnit = unit
-        if (!cancelled(unit)) {
-          currentRun.informUnitStarting(this, unit)
-          task
-        }
-        currentRun.advanceUnit()
+        task
       } finally {
         //assert(currentUnit == unit)
         currentRun.currentUnit = unit0