FIR: only allow member val initialization through this@T

class C {
    val x: Int
    init {
      // valid ways to initialize:
      x = 1
      this@C.x = 1
      // invalid:
      someOtherC.x = 1
      run { /*this@run.*/x = 1 }
      val self = this
      self.x = 1
    }
  }

compiler/testData/diagnostics/tests/controlFlowAnalysis/kt6788.fir.kt

@@ -1,7 +1,7 @@
 class A(val next: A? = null) {
     <!MUST_BE_INITIALIZED_OR_BE_ABSTRACT!>val x: String<!>
     init {
-        next?.x = "a"
+        next?.<!VAL_REASSIGNMENT!>x<!> = "a"
     }
 }
 

compiler/testData/diagnostics/tests/controlFlowAnalysis/propertiesInitWithOtherInstance.fir.kt

@@ -1,12 +1,12 @@
 class A(val next: A? = null) {
     val x: String
     init {
-        next?.x = "a"
+        next?.<!VAL_REASSIGNMENT!>x<!> = "a"
         x = "b"
         this.x = "c"
         x = "d" // don't repeat the same diagnostic again with this receiver
         this.x = "e"
 
-        next?.x = "f"
+        next?.<!VAL_REASSIGNMENT!>x<!> = "f"
     }
 }

compiler/testData/diagnostics/tests/controlFlowAnalysis/propertiesInitWithOtherInstanceInner.fir.kt

@@ -15,7 +15,7 @@ class Outer {
             innerProp = "6" // do not repeat the same diagnostic with this receiver
             this@Inner.innerProp = "7"
 
-            inner.innerProp = "8"
+            inner.<!VAL_REASSIGNMENT!>innerProp<!> = "8"
         }
     }
 }

compiler/testData/diagnostics/tests/controlFlowAnalysis/propertiesInitWithOtherInstanceThisLabel.fir.kt

@@ -1,12 +1,12 @@
 class A(val next: A? = null) {
     val x: String
     init {
-        next?.x = "a"
+        next?.<!VAL_REASSIGNMENT!>x<!> = "a"
         this@A.x = "b"
         this.x = "c"
         x = "d" // don't repeat the same diagnostic again with this receiver
         this@A.x = "e"
 
-        next?.x = "f"
+        next?.<!VAL_REASSIGNMENT!>x<!> = "f"
     }
 }