[clang-tidy][NFC] improve performance misc-unused-using-decls

skip header file before register AST Matchers
it can avoid to matcher lots of ast node when lint header file

Author: HerrCai0907

clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.cpp

@@ -50,6 +50,10 @@ UnusedUsingDeclsCheck::UnusedUsingDeclsCheck(StringRef Name,
       HeaderFileExtensions(Context->getHeaderFileExtensions()) {}
 
 void UnusedUsingDeclsCheck::registerMatchers(MatchFinder *Finder) {
+  // We don't emit warnings on unused-using-decls from headers, so bail out if
+  // the main file is a header.
+  if (utils::isFileExtension(getCurrentMainFile(), HeaderFileExtensions))
+    return;
   Finder->addMatcher(usingDecl(isExpansionInMainFile()).bind("using"), this);
   auto DeclMatcher = hasDeclaration(namedDecl().bind("used"));
   Finder->addMatcher(loc(templateSpecializationType(DeclMatcher)), this);
@@ -82,12 +86,6 @@ void UnusedUsingDeclsCheck::registerMatchers(MatchFinder *Finder) {
 void UnusedUsingDeclsCheck::check(const MatchFinder::MatchResult &Result) {
   if (Result.Context->getDiagnostics().hasUncompilableErrorOccurred())
     return;
-  // We don't emit warnings on unused-using-decls from headers, so bail out if
-  // the main file is a header.
-  if (auto MainFile = Result.SourceManager->getFileEntryRefForID(
-          Result.SourceManager->getMainFileID());
-      utils::isFileExtension(MainFile->getName(), HeaderFileExtensions))
-    return;
 
   if (const auto *Using = Result.Nodes.getNodeAs<UsingDecl>("using")) {
     // Ignores using-declarations defined in macros.