[modules] Further delay calling DeclMustBeEmitted until it's safe.

As discussed in D30793, we have some unsafe calls to isConsumerInterestedIn().
This patch implements Richard's suggestion (from the inline comment) that we
should track if we just deserialized an declaration. If we just deserialized,
we can skip the unsafe call because we know it's interesting. If we didn't just
deserialize the declaration, calling isConsumerInterestedIn() should be safe.

We tried to create a test case for this but we were not successful.

Patch by Raphael Isemann (D32499)!

llvm-svn: 303432