Fix position of class parents

Previously, the position of class parents was the position of the whole
class, which means that NavigateAST#pathTo always returned the class
parents when looking up a position in the body of the class. Fixed by
using the position of the name of the class instead.

Author: smarter

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1312,7 +1312,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
 
     completeAnnotations(cdef, cls)
     val constr1 = typed(constr).asInstanceOf[DefDef]
-    val parentsWithClass = ensureFirstIsClass(parents mapconserve typedParent, cdef.pos.toSynthetic)
+    val parentsWithClass = ensureFirstIsClass(parents mapconserve typedParent, cdef.namePos)
     val parents1 = ensureConstrCall(cls, parentsWithClass)(superCtx)
     val self1 = typed(self)(ctx.outer).asInstanceOf[ValDef] // outer context where class members are not visible
     val dummy = localDummy(cls, impl)