Set module class fields to static except bitmap fields

Still we have a problem with the following magic:

https://github.com/scala/scala/pull/7270/files#r221195225

It is a magic for two reasons:

1. It generates `getstatic` for `Select(This(O$), x)`
2. It is not affected by compilation order of classes

For the second, suppose the backend compiles `A$B` before `A`.
Then the class `A$B` does not see that `x` is static,
which should cause problem at runtime:

```
object A {
  private[this] val x: Int = 10

  class B {
    val y = x
  }
}
```

The following are the reasons why it works in scalac:

1. inner classes always come after the outer class
2. non `private[this]` fields are accessed via accessor methods, which are not static
3. inside the module class, access to the fields are compiled after setting the static flag
4. the code generation specialize for trees like `Select(This(O$), x)`, where `x` is a static member

Author: liufengyun

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -16,6 +16,7 @@ import dotty.tools.dotc.core.Annotations.Annotation
 import dotty.tools.dotc.core.Decorators._
 import dotty.tools.dotc.core.Flags._
 import dotty.tools.dotc.core.StdNames._
+import dotty.tools.dotc.core.NameKinds._
 import dotty.tools.dotc.core.Symbols._
 import dotty.tools.dotc.core.Types._
 import dotty.tools.dotc.core.Contexts._
@@ -129,14 +130,18 @@ trait BCodeSkelBuilder extends BCodeHelpers {
         // reverse all the effects of this transformation, which would be counter-productive.
 
 
-        // TODO: enable once we change lazy val encoding (https://github.com/lampepfl/dotty/issues/7140)
+        // TODO: remove `!f.name.is(LazyBitMapName)` once we change lazy val encoding
+        //       https://github.com/lampepfl/dotty/issues/7140
         //
         // Lazy val encoding assumes bitmap fields are non-static
         //
         // See `tests/run/given-var.scala`
         //
-        // for (f <- claszSymbol.info.decls.filter(_.isField))
-        //   f.setFlag(JavaStatic)
+
+        claszSymbol.info.decls.foreach { f =>
+          if f.isField && !f.name.is(LazyBitMapName) then
+            f.setFlag(JavaStatic)
+        }
 
         val (clinits, body) = impl.body.partition(stat => stat.isInstanceOf[DefDef] && stat.symbol.isStaticConstructor)