Bug 9242: Reuse deviating statement contexts

In the process of deviate add/replace resolution we make
copies of deviated statement contexts which are then added
to the deviation target context.

However, making copies of these statement contexts is not
necessary except the ones which represent unknown statements.

Therefore, reuse them during the resolution of deviate add/replace.

By doing this, we also solve the failing deviate replace with
user-defined types as statement contexts that represent deviating
user-defined types are now resolved within their original context.

Change-Id: Ie61a38e270ef648fe61ea283805d149533c10f62
Signed-off-by: Igor Foltin <igor.foltin@pantheon.tech>