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.
Alternatively, you could just filter out the accessor with `owner.info.decl(name).suchThat(!_.isAccessor)`

Tested out with a little mutation:

```
scala> class C { private val x = 42 }
defined class C

scala> val accessor = typeOf[C].decl(TermName("x"))
accessor: $r.intp.global.Symbol = value x

scala> val accessed = typeOf[C].decl(TermName("x").localName)
accessed: $r.intp.global.Symbol = value x

scala> accessed.name = accessed.name.dropLocal // ala private[this] fields in traits
accessed.name: accessed.NameType = x

scala> typeOf[C].decls.rehash(accessed, accessed.name)

scala> typeOf[C].decl(TermName("x")).alternatives
res41: List[$r.intp.global.Symbol] = List(value x, value x)

scala> typeOf[C].decl(TermName("x")).suchThat(_.isAccessor)
res42: $r.intp.global.Symbol = value x

scala> typeOf[C].decl(TermName("x")).suchThat(_.isAccessor).alternatives
res43: List[$r.intp.global.Symbol] = List(value x)
```