[Clang][NFC] Use std::move to avoid copy (llvm#138073)

Static analysis flagged this code for using copy when we could use
std::move.

Worth noting that CD.Message is a StringRef but Conflict.Message is
std::string. Otherwise I would have used a temporary in place and avoid
a local variable.

Author: shafik

clang/lib/Lex/ModuleMap.cpp

@@ -2010,7 +2010,8 @@ void ModuleMapLoader::handleConflict(const modulemap::ConflictDecl &CD) {
   Conflict.Id = CD.Id;
   Conflict.Message = CD.Message;
 
-  ActiveModule->UnresolvedConflicts.push_back(Conflict);
+  // FIXME: when we move to C++20 we should consider using emplace_back
+  ActiveModule->UnresolvedConflicts.push_back(std::move(Conflict));
 }
 
 void ModuleMapLoader::handleInferredModuleDecl(