[modules] Write out identifiers if the ID is local, too.

In some cases a slot for an identifier is requested but it gets written to
another module, causing an assertion.

At the point when we start serializing Rtypes, we have no imported IdentifierID
for float_round_style. We start serializing stuff and allocate an ID for it.
Then, during the serialization process, we pull in the identifier info for it
from TSchemaHelper. Finally, WriteIdentifierTable decides that the identifier
has not changed since it was deserialized, so doesn't emit it.

Fixes https://llvm.org/bugs/show_bug.cgi?id=27041

Discussed on IRC with Richard Smith. Agreed on post commit review if needed.

llvm-svn: 264913