Mark _metadataAccessorCall with availability

[Azoy]

cc: @jckarter Does this look right?

[aschwaighofer]

I think you also either need a

@available(...) func callAsFunction(request: Int, args: [Any.Type]) -> MetadataResponse { 

or a guard around the accessor call:

#if available(...) _swift_metadataAccessorCall(ptr, request, $0, args.count) 

[jckarter]

Yeah. A good way to get the compiler to help might be to try building the test with an explicit deployment target, like %target-swift-build -target x86_64-apple-macosx10.14.4 %s. Then you should in theory get compiler errors wherever you need availability guards for your new API.