[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/CSDiagnostics.cpp

@@ -6137,3 +6137,45 @@ bool UnableToInferProtocolLiteralType::diagnoseAsError() {
 
   return true;
 }
+
+bool MissingQuialifierInMemberRefFailure::diagnoseAsError() {
+  auto selectedOverload = getOverloadChoiceIfAvailable(getLocator());
+  if (!selectedOverload)
+    return false;
+
+  auto *UDE = cast<UnresolvedDotExpr>(getRawAnchor());
+
+  auto baseType = getType(UDE->getBase());
+
+  auto methodKind = baseType->isAnyExistentialType()
+                        ? DescriptiveDeclKind::StaticMethod
+                        : DescriptiveDeclKind::Method;
+
+  auto choice = selectedOverload->choice.getDeclOrNull();
+  if (!choice)
+    return false;
+
+  auto *DC = choice->getDeclContext();
+  if (!(DC->isModuleContext() || DC->isModuleScopeContext())) {
+    emitDiagnostic(UDE->getLoc(), diag::member_shadows_function, UDE->getName(),
+                   methodKind, choice->getDescriptiveKind(),
+                   choice->getFullName());
+    return true;
+  }
+
+  auto qualifier = DC->getParentModule()->getName();
+
+  emitDiagnostic(UDE->getLoc(), diag::member_shadows_global_function,
+                 UDE->getName(), methodKind, choice->getDescriptiveKind(),
+                 choice->getFullName(), qualifier);
+
+  SmallString<32> namePlusDot = qualifier.str();
+  namePlusDot.push_back('.');
+
+  emitDiagnostic(UDE->getLoc(), diag::fix_unqualified_access_top_level_multi,
+                 namePlusDot, choice->getDescriptiveKind(), qualifier)
+      .fixItInsert(UDE->getStartLoc(), namePlusDot);
+
+  emitDiagnostic(choice, diag::decl_declared_here, choice->getFullName());
+  return true;
+}