user-relevant span: if no frame is in a local crate, use topmost non-track_caller frame

[RalfJung]

Our logic for the user-relevant span ignores all frames that are in non-user-relevant crates. Unfortunately, if the stack consists entirely of frames from non-user-relevant crates, the result is something like this:

    --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1944:9
     |
1944 |         intrinsics::write_via_move(dst, src)
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In other words, we fall back to the topmost frame of the stack. That's not great.

This adjusts the logic so that we instead fall back to the topmost non-track_caller frame. Only if we find no such frame do we use the topmost frame.

This is done by replacing the is_user_relevant: bool with a user_relevance: u8 indicating how relevant the frame is. The current top relevant frame is the topmost frame with the highest relevance. The code that runs on push/pop is adjusted to incrementally update the topmost frame index with the new metric; this should only be marginally more expensive than what we did here before.

r? @saethlin

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.