fix Issue 20688 - Wrong code when linking to C complex number functions

[WalterBright]

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Settle dispute over whether it works or not :-)

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20688	major	Wrong code when linking to C complex number functions

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#11710"

[ibuclaw]

Didn't l fix this months ago?

[WalterBright]

Didn't l fix this months ago?

I don't see how, as it is failing on some platforms!

[ibuclaw]

See #12073, which tests all complex types on all extern linkages.

[ibuclaw]

Ahh, you're testing Windows. I didn't think that they had complex types.