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, 1 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/GenBCode.scala b/src/compiler/scala/tools/nsc/backend/jvm/GenBCode.scala
index 340fdc849a..3520d57599 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/GenBCode.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/GenBCode.scala
@@ -135,7 +135,7 @@ abstract class GenBCode extends BCodeSyncAndTry {
             return
           }
           else {
-            try   { withCurrentUnit(item.cunit)(visit(item)) }
+            try   { withCurrentUnitNoLog(item.cunit)(visit(item)) }
             catch {
               case ex: Throwable =>
                 ex.printStackTrace()