Multiple @nowarn annotations not supported on 2.12

[lrytz]

This works on 2.13

@annotation.nowarn("msg=Auto") @annotation.nowarn("msg=procedure") def f { println }

but 2.12 gives

<console>:11: error: class nowarn may not appear multiple times on method f
       @annotation.nowarn("msg=Auto") @annotation.nowarn("msg=procedure") def f { println }

because on 2.12 nowarn extends ClassfileAnnotation to ensure the argument is a constant. This triggers

https://github.com/scala/scala/blob/v2.12.18/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala#L1481