[clang][flang] Moke the definition of `-module-dir` restricted to Flang

`-module-dir` is a Flang specific option and should not be visible in
Clang. This patch adds `FlangOnlyOption` flag to its definition. This
way Clang will know that it should reject it and skip it when generating
output for `clang -help`.

The definition of `-module-dir` is moved next to other Flang options.
As `-J` is an alias for `-module-dir`, it has to be moved as well (the
alias cannot be defined before the original option). As `gfortran` mode
is effectively no longer supported (*), `-J` is claimed as Flang only
option.

This is a follow-up of a post-commit review for
https://reviews.llvm.org/D95448.

* https://reviews.llvm.org/rG6a75496836ea14bcfd2f4b59d35a1cad4ac58cee

Differential Revision: https://reviews.llvm.org/D99018