Add "generated" to `SymbolTag`

[michaelpj]

Many languages provide the ability for the compiler to generate additional symbols (e.g. Haskell or Rust's deriving). It can be helpful for the language server to talk about these symbols, for example, in a call hierarchy display. However, the fact that the symbols correspond to generated definitions makes them notably different (they might have odd locations, for example), so it would be helpful if clients were able to display some kind of indication that the symbol is a generated one.

SymbolTag seems like a sensible place to put this information.