Use fewer intermediate functions for short backtraces in queries
If we make sure that `compute_fn` in the query's vtable is actually named `__rust_begin_short_backtrace`, we can avoid the need for some additional intermediate functions and stack frames.
This is similar to how the `get_query_incr` and `get_query_non_incr` functions are actually named `__rust_end_short_backtrace`.
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Before/after comparison: https://github.com/rust-lang/rust/pull/151739#issuecomment-3815432527
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- Earlier draft of this PR: https://github.com/rust-lang/rust/pull/151719
- Introduction of this backtrace-trimming: https://github.com/rust-lang/rust/pull/108938
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