[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/analysis-tests/testData/resolve/inference/lambdaInElvis.kt

@@ -0,0 +1,18 @@
+// ISSUE: KT-39012
+
+interface A
+
+fun <T> foo(f: (MutableList<T>) -> Unit): List<T>? = TODO()
+fun <T> listOf(): List<T> = TODO()
+
+fun bar1(w: List<CharSequence>): List<CharSequence>? {
+    return foo { container ->
+        container.add("")
+    } ?: w
+}
+
+fun bar2(): List<CharSequence>? {
+    return foo { container ->
+        container.add("")
+    } ?: listOf()
+}