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/variance/Visibility.fir.kt

@@ -5,11 +5,11 @@ interface Test<in I, out O> {
     <!PRIVATE_PROPERTY_IN_INTERFACE!>private<!> val private_val: I
 
     var interlan_private_set: O
-        private set
+        <!PRIVATE_SETTER_FOR_ABSTRACT_PROPERTY!>private<!> set
     public var public_private_set: O
-        private set
+        <!PRIVATE_SETTER_FOR_ABSTRACT_PROPERTY!>private<!> set
     protected var protected_private_set: O
-        private set
+        <!PRIVATE_SETTER_FOR_ABSTRACT_PROPERTY!>private<!> set
     <!PRIVATE_PROPERTY_IN_INTERFACE!>private<!> var private_private_set: O
         private set
 
@@ -17,4 +17,4 @@ interface Test<in I, out O> {
     public fun public_fun(i: O) : I
     protected fun protected_fun(i: O) : I
     <!PRIVATE_FUNCTION_WITH_NO_BODY!>private<!> fun private_fun(i: O) : I
-}
+}