IR: Rename Metadata::ImplicitCode to SubclassData1, NFC

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.

Differential Revision: https://reviews.llvm.org/D96740