Add `VARIANT` and `PROPVARIANT` support #1934

Job	Run time
Test (stable, x86_64-pc-windows-msvc)	13m 41s
Test (nightly, i686-pc-windows-msvc)	18m 11s
Test (nightly, x86_64-pc-windows-gnu)	17m 17s
Test (stable, i686-pc-windows-gnu)	18m 10s
	1h 7m 19s

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