[flang] Add message formatting for std::int64_t

There is no printf formatting string for std::int64_t. Instead we have
to cast to std::intmax_t and use `%jd`. This change simplifies that by
automatically converting std::int64_t to std::intmax_t when formatting
messages.

Original-commit: flang-compiler/f18@8a2343dfffc9ccb04bdfe4c16ca47128255d47bd
Reviewed-on: https://github.com/flang-compiler/f18/pull/1101
Tree-same-pre-rewrite: false