At the end of a run, close macro runtime's classloader

We can only do this on 2.12.x, because URLClassLoader#close
is new in JDK 7.

Tested manually with the REPL and resident compilers.

```
% qscalac sandbox/macro.scala && (for i in 1 2; do echo sandbox/client.scala; done; printf '\n') | qscalac -Xresident -Ylog:all -Ydebug 2>&1 | grep "Closing macro runtime classloader"
[log terminal] Closing macro runtime classloader
[log terminal] Closing macro runtime classloader

% qscalac sandbox/macro.scala && (for i in 1 2; do echo Macro.m; done; printf '\n') | qscala -Ylog:all -Ydebug 2>&1 | grep "Closing macro runtime classloader"; stty echo
[log terminal] Closing macro runtime classloader
[log terminal] Closing macro runtime classloader
```

Note: this doesn't close handles to JAR files held by the
compiler classpath implementation, that will require changes
elsewhere.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/reflect/scala/reflect/internal/util/ScalaClassLoader.scala b/src/reflect/scala/reflect/internal/util/ScalaClassLoader.scala
index cc5be92489..f3db2017be 100644
--- a/src/reflect/scala/reflect/internal/util/ScalaClassLoader.scala
+++ b/src/reflect/scala/reflect/internal/util/ScalaClassLoader.scala
@@ -139,6 +139,10 @@ object ScalaClassLoader {
       classloaderURLs :+= url
       super.addURL(url)
     }
+    override def close(): Unit = {
+      super.close()
+      classloaderURLs = null
+    }
   }
 
   def fromURLs(urls: Seq[URL], parent: ClassLoader = null): URLClassLoader =