Workaround last issues with -Ytest-pickler

We run checkNoPrivateLeaks in the unpickler since last commit and this
is causing new issues related to privacy leaks. Give up and workaround
them since we're going to redesign how we handle privacy leaks because
of #1723 anyway.

Author: smarter

compiler/src/dotty/tools/dotc/core/Symbols.scala

@@ -574,7 +574,7 @@ object Symbols {
     /** Copy a symbol, overriding selective fields */
     def copy(
         owner: Symbol = sym.owner,
-        name: N = sym.name,
+        name: N = (sym.name: N), // Dotty deviation: type ascription to avoid leaking private sym (only happens in unpickling), won't be needed once #1723 is fixed
         flags: FlagSet = sym.flags,
         info: Type = sym.info,
         privateWithin: Symbol = sym.privateWithin,

compiler/src/dotty/tools/dotc/core/tasty/TreeBuffer.scala

@@ -25,7 +25,7 @@ class TreeBuffer extends TastyBuffer(50000) {
    */
   private val treeAddrs = new java.util.IdentityHashMap[Tree, TreeAddrs]
 
-  def registerTreeAddr(tree: Tree) =
+  def registerTreeAddr(tree: Tree): Unit =
     treeAddrs.put(tree,
       treeAddrs.get(tree) match {
         case null => currentAddr