Serialization: add an option to allow deserializing @_implementationOnly dependencies
In theory, we shouldn't need to deserialize @_implementationOnly dependencies. However,
potential decl recovery issues may bring down lldb if we insist on not importing these
dependencies, resulting in bad user experience as a result. This patch adds an internal
option to allow importing them and it should only be set by lldb and other tools.
rdar://65570721