[ASTImporter] Fix handling of not defined FromRecord in ImportContext(...)

In ImportContext(…) we may call into CompleteDecl(…) which if FromRecrord is not
defined will start the definition of a ToRecord but from what I can tell at least
one of the paths though here don't ensure we complete the definition.
For a RecordDecl this can be problematic since this means we won’t import base
classes and we won’t have any of the methods or types we inherit from these bases.

Differential Revision: https://reviews.llvm.org/D78000