[FIR] Resolve elvis call as special synthetic call

Before that commit we desugared `a ?: b` as

when (val elvis = a) {
    null -> b
    else -> elvis
}

It was incorrect, because `a` should be resolved in dependent mode,
  but when it was `elvis` initializer it was resolved in independent
  mode, so we can't infer type for `a` in some complex cases

compiler/fir/cones/src/org/jetbrains/kotlin/fir/symbols/SyntheticCallableId.kt

@@ -9,20 +9,28 @@ import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 
 object SyntheticCallableId {
+    private val syntheticPackageName: FqName = FqName("_synthetic")
+
     val WHEN = CallableId(
-        FqName("_synthetic"),
+        syntheticPackageName,
         Name.identifier("WHEN_CALL")
     )
     val TRY = CallableId(
-        FqName("_synthetic"),
+        syntheticPackageName,
         Name.identifier("TRY_CALL")
     )
     val CHECK_NOT_NULL = CallableId(
-        FqName("_synthetic"),
+        syntheticPackageName,
         Name.identifier("CHECK_NOT_NULL_CALL")
     )
+
+    val ELVIS_NOT_NULL = CallableId(
+        syntheticPackageName,
+        Name.identifier("ELVIS_CALL")
+    )
+
     val ID = CallableId(
-        FqName("_synthetic"),
+        syntheticPackageName,
         Name.identifier("ID_CALL")
     )
 }