[clang-tidy] Pop Files only if FileChangeReason is ExitFile

enum FileChangeReason has four possible type EnterFile, ExitFile,
SystemHeaderPragma and RenameFile,
It should pop the back element of Files only if FileChangeReason is ExitFile.

Author: tclin914

clang-tools-extra/clang-tidy/readability/DuplicateIncludeCheck.cpp

@@ -71,7 +71,7 @@ void DuplicateIncludeCallbacks::FileChanged(SourceLocation Loc,
                                             FileID PrevFID) {
   if (Reason == EnterFile)
     Files.emplace_back();
-  else
+  else if (Reason == ExitFile)
     Files.pop_back();
 }