[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.

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -1138,14 +1138,12 @@ NOTE(candidate_expected_different_labels,none,
      "incorrect labels for candidate (have: '%0', expected: '%1')",
      (StringRef, StringRef))
 
+ERROR(member_shadows_function,none,
+      "use of %0 refers to %1 rather than %2 %3",
+      (DeclNameRef, DescriptiveDeclKind, DescriptiveDeclKind, DeclName))
 ERROR(member_shadows_global_function,none,
-      "use of %0 refers to %1 %2 rather than %3 %4 in %5 %6",
-      (DeclNameRef, DescriptiveDeclKind, DeclName, DescriptiveDeclKind,
-       DeclName, DescriptiveDeclKind, DeclName))
-ERROR(member_shadows_global_function_near_match,none,
-      "use of %0 nearly matches %3 %4 in %5 %6 rather than %1 %2",
-      (DeclNameRef, DescriptiveDeclKind, DeclName, DescriptiveDeclKind,
-       DeclName, DescriptiveDeclKind, DeclName))
+      "use of %0 refers to %1 rather than %2 %3 in module %4",
+      (DeclNameRef, DescriptiveDeclKind, DescriptiveDeclKind, DeclName, DeclName))
 
 ERROR(instance_member_use_on_type,none,
       "instance member %1 cannot be used on type %0; "

lib/Sema/CSDiagnostics.cpp

@@ -6117,3 +6117,45 @@ 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;
+}

lib/Sema/CSDiagnostics.h

@@ -1936,6 +1936,29 @@ class UnableToInferProtocolLiteralType final : public FailureDiagnostic {
   bool diagnoseAsError();
 };
 
+/// 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.
+class MissingQuialifierInMemberRefFailure final : public FailureDiagnostic {
+public:
+  MissingQuialifierInMemberRefFailure(ConstraintSystem &cs,
+                                      ConstraintLocator *locator)
+      : FailureDiagnostic(cs, locator) {}
+
+  bool diagnoseAsError();
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -1182,7 +1182,9 @@ SpecifyObjectLiteralTypeImport::create(ConstraintSystem &cs,
 }
 
 bool AddQualifierToAccessTopLevelName::diagnose(bool asNote) const {
-  return false;
+  auto &cs = getConstraintSystem();
+  MissingQuialifierInMemberRefFailure failure(cs, getLocator());
+  return failure.diagnose(asNote);
 }
 
 AddQualifierToAccessTopLevelName *