Fixed clang -Wreserved-identifier warings

Created FMT_UNCHECKED_TYPE that resolves to special identifier _Unchecked_type for Microsoft, but to a dummy string otherwise. Using
_Unchecked_type is invalid because underscore + uppercase is a reserved identifier.

include/fmt/format.h

@@ -1968,7 +1968,7 @@ class counting_iterator {
   using difference_type = std::ptrdiff_t;
   using pointer = void;
   using reference = void;
-  using _Unchecked_type = counting_iterator;  // Mark iterator as checked.
+  FMT_UNCHECKED_ITERATOR(counting_iterator);
 
   struct value_type {
     template <typename T> void operator=(const T&) {}