Add `VT_UNKNOWN` support for `VARIANT` and `PROPVARIANT` #1936

Job	Run time
Test (stable, x86_64-pc-windows-msvc)	13m 2s
Test (nightly, i686-pc-windows-msvc)	17m 58s
Test (nightly, x86_64-pc-windows-gnu)	17m 37s
Test (stable, i686-pc-windows-gnu)	18m 57s
	1h 7m 34s

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