KT-18715: don't swallow comments for if->elvis

Adds a CommentSaver to the IfThenToElvis intention that will save and
restore comments when converting an if expression to elvis expression.

idea/src/org/jetbrains/kotlin/idea/intentions/branchedTransformations/intentions/IfThenToElvisIntention.kt

@@ -26,6 +26,7 @@ import org.jetbrains.kotlin.idea.inspections.IntentionBasedInspection
 import org.jetbrains.kotlin.idea.inspections.branchedTransformations.IfThenToSafeAccessInspection
 import org.jetbrains.kotlin.idea.intentions.SelfTargetingOffsetIndependentIntention
 import org.jetbrains.kotlin.idea.intentions.branchedTransformations.*
+import org.jetbrains.kotlin.idea.util.CommentSaver
 import org.jetbrains.kotlin.idea.util.application.runWriteAction
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.resolve.bindingContextUtil.isUsedAsExpression
@@ -78,6 +79,9 @@ class IfThenToElvisIntention : SelfTargetingOffsetIndependentIntention<KtIfExpre
             val ifThenToSelectData = element.buildSelectTransformationData() ?: return
 
             val factory = KtPsiFactory(element)
+
+            val commentSaver = CommentSaver(element, saveLineBreaks = true)
+
             val elvis = runWriteAction {
                 val replacedBaseClause = ifThenToSelectData.replacedBaseClause(factory)
                 val newExpr = element.replaced(
@@ -87,7 +91,10 @@ class IfThenToElvisIntention : SelfTargetingOffsetIndependentIntention<KtIfExpre
                         ifThenToSelectData.negatedClause!!
                     )
                 )
-                KtPsiUtil.deparenthesize(newExpr) as KtBinaryExpression
+
+                (KtPsiUtil.deparenthesize(newExpr) as KtBinaryExpression).also {
+                    commentSaver.restore(it)
+                }
             }
 
             if (editor != null) {

idea/testData/intentions/branched/ifThenToElvis/ifToElvisSwallowComments.kt

@@ -0,0 +1,6 @@
+fun foo(arg: Any): Int {
+    // 1
+    return <caret>if (arg is Int) arg
+    // 2
+    else 10
+}

idea/testData/intentions/branched/ifThenToElvis/ifToElvisSwallowComments.kt.after

@@ -0,0 +1,6 @@
+fun foo(arg: Any): Int {
+    // 1
+    return arg as? Int
+    // 2
+        ?: 10
+}

idea/tests/org/jetbrains/kotlin/idea/intentions/IntentionTestGenerated.java

@@ -2499,6 +2499,11 @@ public class IntentionTestGenerated extends AbstractIntentionTest {
                 runTest("idea/testData/intentions/branched/ifThenToElvis/ifAsPartOfExpression.kt");
             }
 
+            @TestMetadata("ifToElvisSwallowComments.kt")
+            public void testIfToElvisSwallowComments() throws Exception {
+                runTest("idea/testData/intentions/branched/ifThenToElvis/ifToElvisSwallowComments.kt");
+            }
+
             @TestMetadata("implicitReceiver.kt")
             public void testImplicitReceiver() throws Exception {
                 runTest("idea/testData/intentions/branched/ifThenToElvis/implicitReceiver.kt");