[stable] Cherry-pick C++ mangling regression fix from master

[kinke]

From #10477; the regression was introduced in 2.088.1, so let's have 2.089.x fixed and not wait 2 more months for 2.090.x.

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[kinke]

Please merge before v2.089.0 final [my 3 current DMD PRs all fail CI for spurious reasons].

Edit: Oh, already too late, been tagged 7 hours ago. :/

[PetarKirov]

@kinke When you (force) push dlang-bot removes the auto-merge label. Please ping us as soon as you do this, so we can add it back.