Parallelise JVM backend - Scala 2 port (#15392)

This PR ports scala/scala#6124 introduces
backend parallelism, on top of the previously ported changes introduced
in #15322
Adds Scala2 flags: 
* `-Yjar-compression-level:N`
* `-Ybackend-parallelism:N`
* `-Ybackend-worker-queue:N`

Author: bishabosha

Diff for: compiler/src/dotty/tools/backend/jvm/BTypes.scala

@@ -31,8 +31,7 @@ abstract class BTypes { self =>
    * Concurrent because stack map frames are computed when in the class writer, which might run
    * on multiple classes concurrently.
    */
-  protected def classBTypeFromInternalNameMap: collection.concurrent.Map[String, ClassBType]
-    // NOTE: Should be a lazy val but scalac does not allow abstract lazy vals (dotty does)
+  protected lazy val classBTypeFromInternalNameMap: collection.concurrent.Map[String, ClassBType]
 
   /**
    * Obtain a previously constructed ClassBType for a given internal name.

Diff for: compiler/src/dotty/tools/backend/jvm/BTypesFromSymbols.scala

@@ -55,14 +55,15 @@ class BTypesFromSymbols[I <: DottyBackendInterface](val int: I, val frontendAcce
       (classSym != defn.NothingClass && classSym != defn.NullClass),
       s"Cannot create ClassBType for special class symbol ${classSym.showFullName}")
 
-    convertedClasses.getOrElse(classSym, {
-      val internalName = classSym.javaBinaryName
-      // We first create and add the ClassBType to the hash map before computing its info. This
-      // allows initializing cylic dependencies, see the comment on variable ClassBType._info.
-      val classBType = new ClassBType(internalName)
-      convertedClasses(classSym) = classBType
-      setClassInfo(classSym, classBType)
-    })
+    convertedClasses.synchronized:
+      convertedClasses.getOrElse(classSym, {
+        val internalName = classSym.javaBinaryName
+        // We first create and add the ClassBType to the hash map before computing its info. This
+        // allows initializing cylic dependencies, see the comment on variable ClassBType._info.
+        val classBType = new ClassBType(internalName)
+        convertedClasses(classSym) = classBType
+        setClassInfo(classSym, classBType)
+      })
   }
 
   final def mirrorClassBTypeFromSymbol(moduleClassSym: Symbol): ClassBType = {

Diff for: compiler/src/dotty/tools/backend/jvm/BackendUtils.scala

@@ -103,12 +103,10 @@ class BackendUtils(val postProcessor: PostProcessor) {
     // stack map frames and invokes the `getCommonSuperClass` method. This method expects all
     // ClassBTypes mentioned in the source code to exist in the map.
 
-    val serlamObjDesc = MethodBType(jliSerializedLambdaRef :: Nil, ObjectRef).descriptor
-
-    val mv = cw.visitMethod(ACC_PRIVATE + ACC_STATIC + ACC_SYNTHETIC, "$deserializeLambda$", serlamObjDesc, null, null)
+    val mv = cw.visitMethod(ACC_PRIVATE + ACC_STATIC + ACC_SYNTHETIC, "$deserializeLambda$", serializedLamdaObjDesc, null, null)
     def emitLambdaDeserializeIndy(targetMethods: Seq[Handle]): Unit = {
       mv.visitVarInsn(ALOAD, 0)
-      mv.visitInvokeDynamicInsn("lambdaDeserialize", serlamObjDesc, jliLambdaDeserializeBootstrapHandle, targetMethods: _*)
+      mv.visitInvokeDynamicInsn("lambdaDeserialize", serializedLamdaObjDesc, jliLambdaDeserializeBootstrapHandle, targetMethods: _*)
     }
 
     val targetMethodGroupLimit = 255 - 1 - 3 // JVM limit. See See MAX_MH_ARITY in CallSite.java
@@ -133,6 +131,11 @@ class BackendUtils(val postProcessor: PostProcessor) {
     mv.visitInsn(ARETURN)
   }
 
+  private lazy val serializedLamdaObjDesc = {
+    import coreBTypes.{ObjectRef, jliSerializedLambdaRef}
+    MethodBType(jliSerializedLambdaRef :: Nil, ObjectRef).descriptor
+  }
+
   /**
    * Visit the class node and collect all referenced nested classes.
    */