FinalUtils#canBeFinal: field assignments are checked within top-level class

Fixes IDEA-187493 False "Field may be final" inspection

plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/ig/psiutils/FinalUtils.java

@@ -41,7 +41,7 @@ public class FinalUtils {
       return false;
     }
     PsiElement scope = variable instanceof PsiField
-                       ? ((PsiField)variable).getContainingClass()
+                       ? PsiUtil.getTopLevelClass(variable)
                        : PsiUtil.getVariableCodeBlock(variable, null);
     if (scope == null) return false;
     Map<PsiElement, Collection<ControlFlowUtil.VariableInfo>> finalVarProblems = new THashMap<>();

plugins/InspectionGadgets/test/com/siyeh/igtest/style/field_final/FieldMayBeFinal.java

@@ -1071,4 +1071,16 @@ class T74 {
     x = 2;
   }
   int y = x = 3;
+}
+// IDEA-187493
+class T75 {
+  void foo() {
+    new Inner().innerField = 1;
+  }
+
+  private static class Inner {
+    private int innerField;
+
+    private Inner() {innerField = 0;}
+  }
 }