[stable] Fix up 2.087 C++ header regressions

[kinke]

At least those I've encountered so far, incl. a cherry-pick of #10065.

[dlang-bot]

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Testing this PR locally

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dub fetch digger
dub run digger -- build "stable + dmd#10070"

[ibuclaw]

Please add tests, and make it harder for people to blindly break this in future.

Tests added

[thewilsonator]

Mecca failure unrelated.