Float16 for importC

[Emmankoko]

PR is focused on #20700

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Testing this PR locally

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dub run digger -- build "master + dmd#22315"

[WalterBright]

There's a library implementation for float16:

https://github.com/DigitalMars/sargon/blob/master/src/sargon/halffloat.d

[Emmankoko]

Thanks very much @WalterBright , I will go through and see how best it can be used .