Fix FMT_FORMAT_AS const specifier position (fmtlib#1554)

The current `FMT_FORMAT_AS` macro will make `formatter<Char *>::format`
have the first argument type `const Char *&` which is incorrect an
should be `Char *const &`.  This pull request fixes that by changing the
first argument type in the macro definition body from `const Type &` to
`Type const &`.

include/fmt/format.h

@@ -2953,7 +2953,7 @@ struct formatter<T, Char,
   template <typename Char>                                                    \
   struct formatter<Type, Char> : formatter<Base, Char> {                      \
     template <typename FormatContext>                                         \
-    auto format(const Type& val, FormatContext& ctx) -> decltype(ctx.out()) { \
+    auto format(Type const& val, FormatContext& ctx) -> decltype(ctx.out()) { \
       return formatter<Base, Char>::format(val, ctx);                         \
     }                                                                         \
   }