`set XXX` fails for some union data types

[jktjkt]

/> set czechlight-system:journal-upload/host "127.0.0.1"
libyang-cpp internal error: LY_ARRAY_COUNT(lysc_type_union->types) = 3, LY_ARRAY_COUNT(lysp_type->types) = 0, lysp_type->name = inet:host

This affects both command execution as well as tab completion. The root cause is that "a lot of stuff", including parsing of actual values, requires libyang::types::Union::types() to behave reasonably, but that function tries to be too smart when resolving parsed type info in YANG-level unions.

Discovered by @ares1025.