Optional language features don't have a specific "-enable-*" flag, because
they're rare and don't fit the same upcoming/experimental distinction. Add
a flag_name field to provide the flag name as well.
The name of a migratable feature might differ from the names of the
diagnostic groups containing the diagnostics that are used to drive
migration for that feature. Provide the set of diagnostic categories
that are associated with each migratable feature as part of the
supported features list.
This is future-proofing the version part of the upcoming
feature. There currently no features that require that
but it's possible that they'd be added in the future.
This is a replacement for `-emit-supported-features` that prints
all of the upcoming/experimental features supported by the compiler
with some additional meta information in JSON format to stdout.
