Move `describe_lints` calls.

Currently we have an inconsistency between the "input" and "no input"
cases:
- no input: `rustc --print=sysroot -Whelp` prints the lint help.
- input:    `rustc --print=sysroot -Whelp a.rs` prints the sysroot.

It makes sense to print the lint help in both cases, because that's what
happens with `--help`/`-Zhelp`/`-Chelp`.

In fact, the `describe_lints` in the "input" case happens amazingly
late, after *parsing*. This is because, with plugins, lints used to be
registered much later, when the global context was created. But #117649
moved lint registration much earlier, during session construction.

So this commit moves the `describe_lints` call to a single spot for both
for both the "input" and "no input" cases, as early as possible. This is
still not as early as `--help`/`-Zhelp`/`-Chelp`, because `-Whelp` must
wait until the session is constructed.

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