FIR: fix position and reporting of PRIVATE_SETTER_*

Previously `FirPropertyAccessor.source` references the `KtProperty` if
the user code contains a setter or getter that doesn't contain a body.
For example, with the following

```
val i: Int = 1
  private set
```

The `FirPropertyAccessor` would reference the `KtProperty` as the
source.

This change makes `FirPropertyAccessor` reference `KtPropertyAccessor`
as the source if possible, regardless of whether the body is present or
not.

compiler/testData/diagnostics/tests/AbstractAccessor.fir.kt

@@ -1,14 +1,14 @@
 abstract class My {
     abstract var x: Int
         public get
-        private set
+        <!PRIVATE_SETTER_FOR_ABSTRACT_PROPERTY!>private<!> set
 
     abstract val y: Int
         protected get
 
     abstract protected var z: Int
         internal get
-        private set
+        <!PRIVATE_SETTER_FOR_ABSTRACT_PROPERTY!>private<!> set
 
     abstract internal val w: Int
         protected get
@@ -17,5 +17,5 @@ abstract class My {
         protected set
 
     open var t: Int = 0
-        private set
-}
+        <!PRIVATE_SETTER_FOR_OPEN_PROPERTY!>private<!> set
+}