Remove explicit use of Tuple2

This change is needed for scala/scala3#2199.

The idea is to have `type Tuple2[A, B] = (A, B) // Desugars into TupleCons[A, TupleCons[B, Unit]]` in Predef, so that users can refer to "a tuple of size 2" either with `(A, B)` or `Tuple2[A, B]`. It's still possible to refer to the actually underlying class with the explicit prefix, `scala.Tuple2`. Because this code is inside the scala package and mixes the two syntax in breaks with the above PR...

Author: OlivierBlanvillain

src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala

@@ -808,7 +808,7 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
       var flatKeys: List[Int]       = Nil
       var targets:  List[asm.Label] = Nil
       var default:  asm.Label       = null
-      var switchBlocks: List[Tuple2[asm.Label, Tree]] = Nil
+      var switchBlocks: List[(asm.Label, Tree)] = Nil
 
       // collect switch blocks and their keys, but don't emit yet any switch-block.
       for (caze @ CaseDef(pat, guard, body) <- cases) {

src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala

@@ -432,7 +432,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
     var labelDef: scala.collection.Map[Symbol, LabelDef] = null// (LabelDef-sym -> LabelDef)
 
     // bookkeeping the scopes of non-synthetic local vars, to emit debug info (`emitVars`).
-    var varsInScope: List[Tuple2[Symbol, asm.Label]] = null // (local-var-sym -> start-of-scope)
+    var varsInScope: List[(Symbol, asm.Label)] = null // (local-var-sym -> start-of-scope)
 
     // helpers around program-points.
     def lastInsn: asm.tree.AbstractInsnNode = {