IR: Extract set logic from Instruction attachments, NFC

Extract the set logic for metadata attachments from `Instruction` so it
can be reused for `Function` (PR23340).

This data structure makes a `SmallVector<>` look (a little) like a map,
just doing the bare minimum to support the `Instruction` (and soon,
`Function`) metadata API.

llvm-svn: 235769