Warnings in gcc 10

gcc-10 is now available through apt, and is emitting a new warning:

In file included from ../vtkm/internal/testing/UnitTestVariant.cxx:11:
../vtkm/internal/Variant.h: In function ‘void test_variant::TestCopyDestroy()’:
../vtkm/internal/Variant.h:220:5: warning: ‘<anonymous>’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  220 |     this->Storage = std::move(rhs.Storage);

In fact it looks like it could contort itself into a legitimate problem, if happy paths aren't followed. Still, low priority.