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/enum/AbstractInEnum.fir.kt

@@ -11,8 +11,8 @@ enum class MyEnum() {
 
     var b: Int                private set
     var b1: Int = 0;                         private set
-    abstract var b2: Int      private set
-    abstract var b3: Int = <!ABSTRACT_PROPERTY_WITH_INITIALIZER!>0<!>; private set
+    abstract var b2: Int      <!PRIVATE_SETTER_FOR_ABSTRACT_PROPERTY!>private<!> set
+    abstract var b3: Int = <!ABSTRACT_PROPERTY_WITH_INITIALIZER!>0<!>; <!PRIVATE_SETTER_FOR_ABSTRACT_PROPERTY!>private<!> set
 
     var c: Int                set(v: Int) { field = v }
     var c1: Int = 0;                         set(v: Int) { field = v }
@@ -44,4 +44,4 @@ enum class MyEnum() {
     var l1: Int = 0;   abstract get  abstract set
 
     var n: Int                       abstract get abstract set(v: Int) {}
-}
+}