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.

Unsized [out] strings are not allowed because of the potential to
overflow the buffer passed in.

In this case type_array_get_conformance will return NULL. The code
here is actually replacing an existing array with an EXPR_VOID
conformance with the specified conformance, so issue an error if
type_array_get_conformance returns NULL or the conformance has already
been defined.