[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/tests-spec/testData/diagnostics/notLinked/dfa/pos/32.fir.kt

@@ -6,7 +6,7 @@
 fun <T: Any, K: Any> case_1(x: T?, y: K?) {
     x as T
     y as K
-    val z = <!DEBUG_INFO_EXPRESSION_TYPE("T?!! & T?")!>x<!> ?: <!DEBUG_INFO_EXPRESSION_TYPE("K?!!")!>y<!>
+    val z = <!DEBUG_INFO_EXPRESSION_TYPE("T?!! & T?")!>x<!> ?: <!DEBUG_INFO_EXPRESSION_TYPE("K?!! & K?")!>y<!>
 
     <!DEBUG_INFO_EXPRESSION_TYPE("T?!! & T?")!>x<!>.equals(10)
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any")!>z<!>