SLC: force boxed return type when needed

e.g., when overriding a function whose return type is not primitive

^KT-57579 Fixed

compiler/testData/asJava/lightClasses/lightClassByFqName/nullabilityAnnotations/IntOverridesAny.fir.java

@@ -2,10 +2,10 @@ public final class C /* C*/ implements Tr {
   private final int v;
 
   @java.lang.Override()
-  public int foo();//  foo()
+  public java.lang.Integer foo();//  foo()
 
   @java.lang.Override()
-  public int getV();//  getV()
+  public java.lang.Integer getV();//  getV()
 
   public  C();//  .ctor()
 }