Auto merge of #115391 - saethlin:depkind-discrim, r=nnethercote

Encode DepKind as u16

The derived Encodable/Decodable impls serialize/deserialize as a varint, which results in a lot of code size around the encoding/decoding of these types which isn't justified: The full range of values here is rather small but doesn't quite fit in to a `u8`. Growing _all_ serialized `DepKind` to 2 bytes costs us on average 1% size in the incr comp dep graph, which I plan to recoup in rust-lang/rust#110050 by taking advantage of the unused bits in all the serialized `DepKind`.

r? `@nnethercote`

Author: bors