Always pickle the position of DefTrees

A symbol is created before the corresponding tree is unpickled,
so we cannot rely on initialPos to set the symbol position.
Instead, we always pickle the position of definitions.

In practice, this means that we now always pickle the position of all
`TypeDef` and `Bind` (the other `DefTree`s are all `WithLazyField` and
thus already had their positions pickled).

Author: smarter

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

@@ -41,11 +41,21 @@ class PositionPickler(pickler: TastyPickler, addrOfTree: tpd.Tree => Option[Addr
       lastPos = pos
     }
 
-    /** True if x's position cannot be reconstructed automatically from its initialPos
+    /** True if x's position shouldn't be reconstructed automatically from its initialPos
      */
     def alwaysNeedsPos(x: Positioned) = x match {
-      case _: WithLazyField[_]            // initialPos is inaccurate for trees with lazy field
-         | _: Trees.PackageDef[_] => true // package defs might be split into several Tasty files
+      case
+          // initialPos is inaccurate for trees with lazy field
+          _: WithLazyField[_]
+
+          // A symbol is created before the corresponding tree is unpickled,
+          // and its position cannot be changed afterwards.
+          // so we cannot use the tree initialPos to set the symbol position.
+          // Instead, we always pickle the position of definitions.
+          | _: Trees.DefTree[_]
+
+          // package defs might be split into several Tasty files
+          | _: Trees.PackageDef[_] => true
       case _ => false
     }