[clang-doc] Prevent copying loop variables (NFC)

/llvm-project/clang-tools-extra/clang-doc/HTMLMustacheGenerator.cpp:86:19:
error: loop variable '[Name, FileName]' creates a copy from type 'std::pair<llvm::StringRef, llvm::StringRef> const' [-Werror,-Wrange-loop-construct]
  for (const auto [Name, FileName] : Partials)
                  ^
/llvm-project/clang-tools-extra/clang-doc/HTMLMustacheGenerator.cpp:86:8:
note: use reference type 'std::pair<llvm::StringRef, llvm::StringRef> const &' to prevent copying
  for (const auto [Name, FileName] : Partials)
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                  &
1 error generated.

Author: DamonFool

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

@@ -83,7 +83,7 @@ setupTemplate(std::unique_ptr<MustacheTemplateFile> &Template,
   if (Error Err = T.takeError())
     return Err;
   Template = std::move(T.get());
-  for (const auto [Name, FileName] : Partials)
+  for (const auto &[Name, FileName] : Partials)
     if (auto Err = Template->registerPartialFile(Name, FileName))
       return Err;
   return Error::success();