[Diagnostics] Port name shadowing diagnostics

Diagnose an attempt to reference a top-level name shadowed by
a local member e.g.

```swift
extension Sequence {
  func test() -> Int {
    return max(1, 2)
  }
}
```

Here `min` refers to a global function `min<T>(_: T, _: T)` in `Swift`
module and can only be accessed by adding `Swift.` to it, because `Sequence`
has a member named `min` which accepts a single argument.

lib/Sema/CSFix.cpp

@@ -1219,7 +1219,9 @@ AllowNonClassTypeToConvertToAnyObject::create(ConstraintSystem &cs, Type type,
 }
 
 bool AddQualifierToAccessTopLevelName::diagnose(bool asNote) const {
-  return false;
+  auto &cs = getConstraintSystem();
+  MissingQuialifierInMemberRefFailure failure(cs, getLocator());
+  return failure.diagnose(asNote);
 }
 
 AddQualifierToAccessTopLevelName *