SI-8087 keep annotations on mixed-in private[this] fields

Related to SI-2511 / eea7956, which fixed the same issue for non
`private[this]` fields.

If you have

    trait T { private[this] val f = 0 }
    class C extends T

Mixin geneartes an accessor method `T.f` with owner `T`. When
generating the field in `C`, the Mixin.mixinTraitMembers calls
`fAccessor.accessed`. The implementation of `accessed` does a lookup
for a member named `"f "` (note the space). The bug is that
`private[this]` fields are not renamed to have space
(`LOCAL_SUFFIX_STRING`) in their name, so the accessed was not found,
and no annotations were copied from it.
I tend to think it would be simpler to test this using Java reflection, rather than with ASM.

```scala
scala> trait T { @volatile private[this] var x = 42 }; class C extends T
defined trait T
defined class C

scala> java.lang.reflect.Modifier.isVolatile(classOf[C].getDeclaredFields.head.getModifiers)
res24: Boolean = false

scala> class C { @volatile private[this] var x = 42 }
defined class C

scala> java.lang.reflect.Modifier.isVolatile(classOf[C].getDeclaredFields.head.getModifiers)
res25: Boolean = true
```