add a --dep-file option

[froydnj]

Some projects need/want finer-grained control over where dependency
information is generated, rather than accepting the rustc default. For
those projects, add a --dep-file option which enables them to specify
the location of the dependency info file. It looks like config::Options
had such support already, so all that needs to be done is adding the
appropriate option and stuffing the information into the options
structure.

Fixes #28716.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

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[froydnj]

It's worth noting that src/librustc_driver/driver.rs:write_out_deps references a --dep-file option, so perhaps one was present in the past, and got removed in some code shuffling?

[froydnj]

Looks like #28768 overlaps this pull request and does a better job, so closing this one in favor of that.