crypt32: Use an empty string as a separator when no separator is desired to avoid special cases for NULL.

This mirrors Dan Kegel's fix for iphlpapi.

dxdiagn: Successfully copy to destination variants with an invalid type in IDxDiagContainer::GetProp.

GetProp now simply unconditionally clears the destination variant if
VariantClear fails.

widl: Check whether an expression is a constant one before assuming it is a toplevel conformance when writing conformance or variance descriptors.

This allows the code to not have to track the operator type twice.

It's only set, never read from.

It was just a simple indirection to get to a var_t, so just replace
all uses of it with the latter.