In a compiler built with debug extensions, pass `--debug-incremental` to
spawn the "incremental debug server". This is a TCP server exposing a
REPL which allows querying a bunch of compiler state, some of which is
stored only when that flag is passed. Eventually, this will probably
move into `std.zig.Server`/`std.zig.Client`, but this is easier to work
with right now. The easiest way to interact with the server is `telnet`.
