Mark TypeBuilder.DefinePInvokeMethod as RequiresUnreferencedCode

[MichalStrehovsky]

#45633 (comment)

If linker didn't see the p/invoke, it would not have done the necessary work to ensure the generated marshalling code for the p/invoke can work (constructors might be missing for types used in out positions of the signature, the p/invoke might introduce COM into the process etc.).

We should put RequiresUnreferencedCodeAttribute on all overloads in the ref assembly and all the implementations.

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