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/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
+}