Tolerate unresolvable leafrefs in groupings

Tolerate unresolvable leafrefs in groupings

In case a grouping contains a relative leafref pointing outside of
the grouping subtree we cannot safely determine the target type, which
lead to an IllegalArgumentException.

In this case we need to resolve the return type to an Object, which
loses type safety, but really is the best we can do as we just do not
know in what contexts that grouping is used.

Unfortunately this requires API changes to mdsal-binding-generator-api
as we need to communicate the fact we are resolving a type definition
in the context of a grouping to TypeProvider implementations.

JIRA: MDSAL-182
Change-Id: Iaf96d133c08eb47f4bb27b6c4f3db1463b78f05e
Signed-off-by: Vratko Polak <vrpolak@cisco.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>