Allow soft keywords open and infix under -Xsource:3

[smarter]

Since everything is open and can be used infix by default in Scala 2,
these keywords are no-op, but they're useful for cross-compiling with a
future version of Scala 3 where they will be required in some
cases (with Scala 3.0 they're only required to avoid warnings under
-source future).

See https://dotty.epfl.ch/docs/reference/changed-features/operators.html
and http://dotty.epfl.ch/docs/reference/other-new-features/open-classes.html
for details.

[lrytz]

There's a change of semantics here, this used to run the infix method:

scala> class C {
     |   def infix = println("hi")
     |   infix
     |   def f = 1
     | }
class C

scala> new C
val res0: C = C@7b451bf4

Is there an easy way to issue an error here? Or is it exotic enougth to be ignored?

[smarter]

I'd say that's an inevitable semantic change, emitting a warning or error would be very tricky since it would require both token-level information and name resolution

[lrytz]

👍 thanks for the rebase