Implement P0738R2 istream_iterator cleanup

[miscco]

Description

This addresses #35 implementing P0738R2

I went for the ugly solution with an _HAS_CXX20 ifdef. The cleaner solution would certainly be to utilize member initialization and simply default the default constructor. However I assume that this is not a backward compatible change.

Checklist

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[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

Azure Pipelines claims that <iterator> needs to be clang-formatted. (#255 will improve our diagnostics, telling us what code needs to be changed.)

[StephanTLavavej]

Casey approved this in the MSVC-internal repo where it has been successfully committed, so I'm going to go ahead and merge this here with administrator powers.

[StephanTLavavej]

Thanks for the feature implementation! This will show up in the next update of our status chart :-)