[reflection][fix][msvc]: warning C4127 (#865)

Author: MiroKaku

include/ylt/reflection/template_string.hpp

@@ -36,7 +36,7 @@ inline constexpr std::string_view type_string() {
   constexpr auto next1 = str.rfind(sample[pos + 3]);
 #if defined(_MSC_VER)
   constexpr std::size_t npos = str.find_first_of(" ", pos);
-  if (npos != std::string_view::npos)
+  if constexpr (npos != std::string_view::npos)
     return str.substr(npos + 1, next1 - npos - 1);
   else
     return str.substr(pos, next1 - pos);