[FIR] Get rid of candidateSymbol in FirNamedReference

`candidateSymbol` has any reasonable meaning only for references with
  not completed candidate, so this property is moved from FirNamedReference
  to new node FirNamedReferenceWithCandidateSymbol, which has real
  implementation only in :resolve module (`FirNamedReferenceWithCandidate`)

plugins/assign-plugin/assign-plugin.k2/src/org/jetbrains/kotlin/assignment/plugin/k2/FirAssignmentPluginAssignAltererExtension.kt

@@ -67,7 +67,6 @@ class FirAssignmentPluginAssignAltererExtension(
             calleeReference = buildSimpleNamedReference {
                 source = variableAssignment.source
                 name = ASSIGN_METHOD
-                candidateSymbol = null
             }
             origin = FirFunctionCallOrigin.Regular
         }