[Serialization] Fix deserialization crash occurring when a mixed framework fails to load its ObjC part.

[akyrtzi]

Reviewed by Jordan Rose.

Fixes crash with a case such as:

• there is mixed framework A
• swift module B depends on A and is built fine
• there is a swift invocation that imports B but causes the ObjC part of A to fail to import
• trying to serialize decls of B that depend on A will crash

rdar://27709042

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