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.

src/reflect/scala/reflect/internal/Symbols.scala

@@ -2023,12 +2023,19 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
       info.decls.filter(sym => !sym.isMethod && sym.isParamAccessor).toList
 
     /** The symbol accessed by this accessor (getter or setter) function. */
-    final def accessed: Symbol = accessed(owner.info)
-
-    /** The symbol accessed by this accessor function, but with given owner type. */
-    final def accessed(ownerTp: Type): Symbol = {
+    final def accessed: Symbol = {
       assert(hasAccessorFlag, this)
-      ownerTp decl localName
+      val localField = owner.info decl localName
+
+      if (localField == NoSymbol && this.hasFlag(MIXEDIN)) {
+        // SI-8087: private[this] fields don't have a `localName`. When searching the accessed field
+        // for a mixin accessor of such a field, we need to look for `name` instead.
+        // The phase travel ensures that the field is found (`owner` is the trait class symbol, the
+        // field gets removed from there in later phases).
+        enteringPhase(picklerPhase)(owner.info).decl(name).suchThat(!_.isAccessor)
+      } else {
+        localField
+      }
     }
 
     /** The module corresponding to this module class (note that this

test/files/run/t8087.scala

@@ -0,0 +1,12 @@
+trait Foo {
+  @volatile private[this] var x: String = ""
+  @volatile private var y: String = ""
+}
+
+class Bar extends Foo
+
+object Test extends App {
+  classOf[Bar].getDeclaredFields.foreach(f => {
+    assert(java.lang.reflect.Modifier.isVolatile(f.getModifiers), f.getName)
+  })
+}