Report error when language feature is invalid in -language

[adpi2]

Minimized code

dotc -language:foo Hello.scala

Output

Compile successfuly

Expectation

Under the same circumstances, scalac fails with the following error:

'foo' is not a valid choice for '-language'

dotc should have the same behavior or it should at least report a warning.

[som-snytt]

Scala 2 has a MultiChoiceSetting for this purpose, where multiple choices from an enum can be selected.

[odersky]

That's going to be quite messy to keep in lockstep with language imports.

[sjrd]

@odersky It seems @som-snytt found a way to derive them automatically from available language imports, so that there will be no problem keeping them in sync.

[som-snytt]

It's pretty messy. There is a similar TODO in Scala 2. For the chicken/egg issue, an alternative would be to verify settings after the compiler is constructed. As a random example, verifying a phase option names a phase from a plugin is possible only later.