Metadata::ImplicitCode is a bit shaved off of Metadata::Storage,
currently only in use by the subclass DILocation. However, the bit isn't
reserved for that purpose. Rename it SubclassData1 to make it clear
that it has nothing to do with Metadata itself (and other subclasses are
free to use it).
As a drive-by, remove an old TODO about exposing bits to subclasses
(looks like that has mostly been done).
No functionality change here.