Add error when suspension has jar output

To avoid a compiler crash.

Author: nicolasstucki

compiler/src/dotty/tools/backend/jvm/GenBCode.scala

@@ -55,6 +55,10 @@ class GenBCode extends Phase {
     try super.runOn(units)
     finally myOutput match {
       case jar: JarArchive =>
+        if (ctx.run.suspendedUnits.nonEmpty)
+          // If we close the jar the next run will not be able to write on the jar.
+          // But if we do not close it we cannot use it as part of the macro classpath of the suspended files.
+          ctx.error("Can not suspend and output to a jar at the same time. See suspension with -Xprint-suspension.")
         jar.close()
       case _ =>
     }