Suppress warning about missing noreturn attribute

Adding `[[noreturn]]` to `report_unknown_type` suppresses the Clang/GCC `-Wmissing-noreturn` warning:

Clang outputs:

    .../fmt/fmt/format.cc:294:74: warning:
          function 'report_unknown_type' could be declared with
          attribute 'noreturn' [-Wmissing-noreturn]
      ...code, const char *type) {
                                 ^

GCC outputs:

    .../fmt/fmt/format.cc:294:74: warning: function might be candidate for
        attribute 'noreturn' [-Wsuggest-attribute=noreturn]
      ...code, const char *type) {
                                 ^

Cherry-picked d16c4d.