Print a backtrace when query forcing fails. The aim of this PR is to help debugging incremental compilation bugs where query forcing panics. For instance: https://github.com/rust-lang/rust/issues/90682 https://github.com/rust-lang/rust/issues/90697 https://github.com/rust-lang/rust/issues/90715 https://github.com/rust-lang/rust/issues/90739 https://github.com/rust-lang/rust/issues/91401 These bugs happen when the dep-graph attempts to force a dep-node whose fingerprint does not correspond to an actual DefPathHash. PR https://github.com/rust-lang/rust/pull/91741 attempts to hide this bug. I still don't know how to reproduce these bugs, so I sadly could not test this debugging device.