dtoh: Emit placeholders for skipped virtual functions

[MoonlightSentinel]

This ensures proper vtable layout for classes which have extern(D) functions.

[kinke]

Thx, that fixes https://issues.dlang.org/show_bug.cgi?id=21271.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21271	normal	C++ header generation ignores extern(D) class methods affecting vtable layout

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#11872"

[ibuclaw]

Can we add a Fix Issue 21271 in the commit message for the auto-close?

[MoonlightSentinel]

Sure