[MU4] Fix #312489 Harmony duration interpretation

[jeetee]

Resolves: https://musescore.org/en/node/312489

Cherry pick of PR #6857 from 3.x to master

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[jeetee]

Here is a perfect example of the lexical wrongness of pointer notation combined with const:
Original code= Measure const * abc (= a pointer to a const Measure)
"Corrected"= Measure const* abc (= quickly misgrouped and misread as a const* to a Measure, which it isn't)