Fix renaming of synthetic positions in IDE

While working on the IDE tests, I noticed that the Dotty IDE didn't
behave as I expected in the following scenario:

  // Rename `Foo` to `Bar`
  class Foo { new Foo }

The Dotty IDE would consider that 3 places need changing, instead of
only 2:

 - The `Ident(Foo)` in the `TypeDef`
 - The `Ident(Foo)` in the constructor
 - The `Ident(Foo)` in `Select(new Foo, <init>)`

The third tree, however is synthetic: it doesn't need to be changed in
the editor.

Trees whose positions are not derived from the source are now excluded
when doing a rename.

Author: Duhemm

language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala

@@ -303,8 +303,9 @@ class DottyLanguageServer extends LanguageServer
       val newName = params.getNewName
 
       val refs = Interactive.namedTrees(trees, includeReferences = true, tree =>
-        (Interactive.matchSymbol(tree, sym, Include.overriding)
-          || (linkedSym != NoSymbol && Interactive.matchSymbol(tree, linkedSym, Include.overriding))))
+        tree.pos.isSourceDerived
+          && (Interactive.matchSymbol(tree, sym, Include.overriding)
+            || (linkedSym != NoSymbol && Interactive.matchSymbol(tree, linkedSym, Include.overriding))))
 
       val changes = refs.groupBy(ref => toUri(ref.source).toString).mapValues(_.map(ref => new TextEdit(range(ref.namePos), newName)).asJava)