This PR fixes#12846, where the new do elaborator produced confusing
errors when a do element's continuation had a mismatched monadic result
type. The errors were misleading both in location (e.g., pointing at the
value of `let x ← value` rather than the `let` keyword) and in content
(e.g., mentioning `PUnit.unit` which the user never wrote).
The fix introduces `DoElemCont.ensureUnitAt`/`ensureHasTypeAt`, which
check the continuation result type early and report mismatches with a
clear message ("The `do` element has monadic result type ... but the
rest of the `do` block has monadic result type ..."). Each do-element
elaborator (`let`, `have`, `let rec`, `for`, `unless`, `dbg_trace`,
`assert!`, `idbg`, etc.) now captures its keyword token via `%$tk` and
passes it to `ensureUnitAt` so that the error points at the do element
rather than at an internal elaboration artifact. The old ad-hoc type
check in `for` and the confusing `ensureHasType` call in
`continueWithUnit` are replaced by this uniform mechanism. Additionally,
`extractMonadInfo` now calls `instantiateMVars` on the expected type,
and `While.lean`/`If.lean` macros propagate token info through their
expansions.
Closes#12846
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Co-authored-by: Rob23oba <robin.arnez@web.de>
