Expanding cli args with `dotc @file.txt` is unsupported

[olafurpg]

$ cat foo.args
-help
$ scalac @foo.args
Usage: scalac <options> <source files>
where possible standard options include:
  -Dproperty=value                     Pass -Dproperty=value directly to the runtime system.
...
$ dotc @foo.args
Exception in thread "main" java.lang.UnsupportedOperationException: class dotty.tools.dotc.config.CompilerCommand$.expandArg
	at dotty.tools.dotc.util.DotClass.unsupported(DotClass.scala:10)

It would be nice if expanding args from a file was possible. This feature is used with scalac to run the compiler benchmark on real-world projects, see scala/scala-dev#392

See:
https://github.com/lampepfl/dotty/blob/8ddfcaf1667b8972e70e769feffef240ee0bbc10/compiler/src/dotty/tools/dotc/config/CompilerCommand.scala#L40

[liufengyun]

I'm looking at this. Will propose a PR soon.