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"
     }
 }