Remove Drop impl on VARIANT and PROPVARIANT

[riverar]

VARIANT and PROPVARIANT previously implemented Drop by unconditionally calling VariantClear/PropVariantClear. This created a problem as ownership of these types is context-dependent. In particular, some implementations of COM interfaces (e.g., IUIAutomationPropertyChangedEventHandler) receive [in] VARIANT parameters they do not own, and the implicit Drop would free memory still owned by the caller, causing double-frees and spurious Release calls on embedded values.

These types now implement Free instead, consistent with other Win32 types whose cleanup is context-dependent. Callers that require automatic cleanup should wrap values in Owned<T>:

{
  let v = Owned::new(VARIANT::from(my_bstr));
}
// VariantClear is called automatically

Fixes #3818

[riverar]

I considered special-casing the [in] VARIANT parameters to emit *const VARIANT on x64/ARM64 and ManuallyDrop<VARIANT> on x86 (keying off DoNotReleaseAttribute) but this was abandoned because the metadata attribute isn't consistently applied and code-gen complexity would increase (i.e., #[cfg(target_arch = "x86")] all over the place).

[riverar]

Manually verified #3818 continues to crash (cargo run --release) with 0.62.2 on Windows 26300.7760; switched to this branch, no longer crashes.