Do the implicit search shadowing check in the correct context

This commit fixes a very sneaky bug, the following code:
```
lazy val shadowing =
  typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)
       (nestedContext.addMode(Mode.ImplicitShadowing).setExploreTyperState)
```
is parsed by scalac as:
```
lazy val shadowing =
  typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto);
(nestedContext.addMode(Mode.ImplicitShadowing).setExploreTyperState);
```
So we don't actually use the nested context in `typed`, instead we end
up implicitly using `ctx`!

Author: smarter

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

@@ -491,8 +491,8 @@ trait Implicits { self: Typer =>
             pt)
         val generated1 = adapt(generated, pt)
         lazy val shadowing =
-          typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)
-               (nestedContext.addMode(Mode.ImplicitShadowing).setExploreTyperState)
+          typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)(
+            nestedContext.addMode(Mode.ImplicitShadowing).setExploreTyperState)
         def refMatches(shadowing: Tree): Boolean =
           ref.symbol == closureBody(shadowing).symbol || {
             shadowing match {