Test that @_implements() diagnoses module selector

Author: beccadax

lib/Parse/ParseDecl.cpp

@@ -1138,7 +1138,7 @@ Parser::parseImplementsAttribute(SourceLoc AtLoc, SourceLoc Loc) {
     return Status;
   }
 
-  // FIXME(ModQual): Reject module qualification on MemberName.
+  assert(MemberName.getModuleSelector().empty());
   return ParserResult<ImplementsAttr>(
     ImplementsAttr::create(Context, AtLoc, SourceRange(Loc, rParenLoc),
                            ProtocolType.get(), MemberName.getFullName(),

test/NameLookup/module_selector.swift

@@ -474,3 +474,8 @@ func badModuleNames() {
   let y: A.NonexistentModule::MyChildType = fatalError()
   // expected-error@-1 {{'NonexistentModule::MyChildType' is not a member type of struct 'ModuleSelectorTestingKit.A'}}
 }
+public struct BadImplementsAttr: CustomStringConvertible {
+  @_implements(CustomStringConvertible, Swift::description)
+  // expected-error@-1 {{name cannot be qualified with module selector here}} {{41-48=}}
+  public var stringValue: String { fatalError() }
+}