rustc_errors: preserve order of stashed diagnostics when stealing

[kn1g78]

Fix the ordering of stashed diagnostics when one of them is stolen.

stashed_diagnostics is an order-preserving map. However, the diagnostic
stealing helpers currently use swap_remove, which moves the last entry
into the position of the removed entry.

As a result, stealing a diagnostic can unexpectedly reorder the remaining
stashed diagnostics.

Use shift_remove instead, which removes the requested entry while
preserving the order of all remaining entries.

Add a regression test covering this behavior by stashing diagnostics
A, B, and C, stealing A, and verifying that the remaining
diagnostics are emitted in the original order:

B
C

[rustbot]

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[rust-bors]

☔ The latest upstream changes (presumably #163111) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.