Fix position of implicit conversions for IDEs

In f24190d the position was changed to
be zero-extent so that hover-on-type and other IDE features would work
correctly for expressions that get implicitly converted.
In 8a60a20 this got reverted without
explanation, I assume that this was an error so I'm now restoring it.

Author: smarter

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

@@ -773,7 +773,7 @@ trait Implicits { self: Typer =>
       def typedImplicit(cand: Candidate)(implicit ctx: Context): SearchResult = track("typedImplicit") { ctx.traceIndented(i"typed implicit ${cand.ref}, pt = $pt, implicitsEnabled == ${ctx.mode is ImplicitsEnabled}", implicits, show = true) {
         assert(constr eq ctx.typerState.constraint)
         val ref = cand.ref
-        var generated: Tree = tpd.ref(ref).withPos(pos)
+        var generated: Tree = tpd.ref(ref).withPos(pos.startPos)
         if (!argument.isEmpty)
           generated = typedUnadapted(
             untpd.Apply(untpd.TypedSplice(generated), untpd.TypedSplice(argument) :: Nil),