Rollup merge of #140969 - Stypox:logger-layer, r=RalfJung,oli-obk

Allow initializing logger with additional tracing Layer

This PR adds functions to the `rustc_log` and `rustc_driver_impl` crates to allow initializing the logger with an additional `tracing_subscriber::Layer`. This will be used in Miri to save trace events to file using e.g. [`tracing-chrome`](https://github.com/thoren-d/tracing-chrome)'s or [`tracing-tracy`](https://github.com/nagisa/rust_tracy_client)'s `Layer`s.

Additional context on the choice of signature can be found in [this Zulip thread](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Adding.20a.20dependency.20to.20Miri.20that.20depends.20on.20a.20rustc.20dep/near/515707776).

I re-exported `tracing_subscriber::{Layer, Registry};` from `rustc_log` so that `rustc_driver_impl` can use them in the function signature without depending on `tracing_subscriber` directly. I did this to avoid copy-pasting the dependency line with all of the enabled features from the `rustc_log` to the `rustc_driver_impl`'s Cargo.toml, which would have possibly led to redundancies and inconsistencies.

r? `@RalfJung`

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