Move invocation_temp into OutputFilenames

[bjorn3]

While it was previously defined in Session, it is only ever used with OutputFilenames methods.

[rustbot]

The GCC codegen subtree was changed

cc @antoyo, @GuillaumeGomez

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[rustbot]

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