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Attempt to fix build with MSVC.

Authored by rsmith on Apr 15 2020, 1:26 PM.

Description

Attempt to fix build with MSVC.

MSVC appears to instantiate the virtual members of FoldingSet when
instantiating the class definition, thereby requiring the element type
to be defined so that its hash function is known.

This is intended to be a temporary fix; ideally, FoldingSet should not
require this.