[clang-doc][NFC] Use isa<> over dyn_cast (#136445)

Author: ilovepi

clang-tools-extra/clang-doc/Serialize.cpp

@@ -246,9 +246,9 @@ static TypeInfo getTypeInfoForType(const QualType &T,
     return TypeInfo(Reference(SymbolID(), T.getAsString(Policy)));
 
   InfoType IT;
-  if (dyn_cast<EnumDecl>(TD)) {
+  if (isa<EnumDecl>(TD)) {
     IT = InfoType::IT_enum;
-  } else if (dyn_cast<RecordDecl>(TD)) {
+  } else if (isa<RecordDecl>(TD)) {
     IT = InfoType::IT_record;
   } else {
     IT = InfoType::IT_default;