Also deprecate backing field symbols.

Compiling
    @deprecated val foo: T = some.deprecated(call)
yielded
    private[this] val `foo `: T = some.deprecated(call)
    @deprecated <accessor> def foo: T = this.`foo `
where the `@deprecated` has been slapped on the def (where it'll incur
deprecation warnings on callers) but not on the val (where it'll
suppress deprecation warnings on the body.

Just copy the annotation across.

Fixes scala/bug#11538 in an expedient manner.

Author: hrhino

library/src/scala/deprecated.scala

@@ -64,5 +64,5 @@ import scala.annotation.meta._
  *  @see    [[scala.deprecatedOverriding]]
  *  @see    [[scala.deprecatedName]]
  */
-@getter @setter @beanGetter @beanSetter
+@getter @setter @beanGetter @beanSetter @field
 class deprecated(message: String = "", since: String = "") extends scala.annotation.StaticAnnotation