BitcodeWriter: Move abbreviation for GenericDINode; almost NFC

Simplify ValueEnumerator and WriteModuleMetadata by shifting the logic
for the METADATA_GENERIC_DEBUG abbreviation into WriteGenericDINode.
(This is just like r264302, but for GenericDINode.)

The only change is that the abbreviation is emitted later in the
bitcode, just before the first `GenericDINode` record.  This shouldn't
be observable though.

llvm-svn: 264303