Make ocamlc/ocamlopt fail when not given input files

[MisterDA]

An invocation such as ocamlc (without any command-line parameters), or ocamlc -o a.out will now fail with an error message and an error code.

cc @shindere who reviewed the PR offline
fixes #12497

[shindere]

Thanks! I indeed reviewed but would like this to also be reviewed by somebody else. Especially, changing the behaviour of the compiler when it is passed no arguemnt atall was not part of the initial plan so it'd be good to make sure this change is wanted, too.

[MisterDA]

Rebased to fix conflicts in Changes.

[shindere]

This has been discussed in triaging meeting and it has been agreed that
the behaviour of failing when no input file is provided is what we want, even
when no -o foo option is specified.

This PR should be good to go once it has been rebased to solve the conflict
in Changes.

[xavierleroy]

As discussed during today's triage meeting: the "no input files" error behavior looks good, that's what GCC, Clang and CompCert do.