[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()
+}

compiler/fir/analysis-tests/testData/resolve/inference/lambdaInElvis.txt

@@ -0,0 +1,21 @@
+FILE: lambdaInElvis.kt
+    public abstract interface A : R|kotlin/Any| {
+    }
+    public final fun <T> foo(f: R|(kotlin/collections/MutableList<T>) -> kotlin/Unit|): R|kotlin/collections/List<T>?| {
+        ^foo R|kotlin/TODO|()
+    }
+    public final fun <T> listOf(): R|kotlin/collections/List<T>| {
+        ^listOf R|kotlin/TODO|()
+    }
+    public final fun bar1(w: R|kotlin/collections/List<kotlin/CharSequence>|): R|kotlin/collections/List<kotlin/CharSequence>?| {
+        ^bar1 R|/foo|<R|kotlin/CharSequence|>(<L> = foo@fun <anonymous>(container: R|kotlin/collections/MutableList<kotlin/CharSequence>|): R|kotlin/Unit| {
+            ^ R|<local>/container|.R|FakeOverride<kotlin/collections/MutableList.add: R|kotlin/Boolean|>|(String())
+        }
+        ) ?: R|<local>/w|
+    }
+    public final fun bar2(): R|kotlin/collections/List<kotlin/CharSequence>?| {
+        ^bar2 R|/foo|<R|kotlin/CharSequence|>(<L> = foo@fun <anonymous>(container: R|kotlin/collections/MutableList<kotlin/CharSequence>|): R|kotlin/Unit| {
+            ^ R|<local>/container|.R|FakeOverride<kotlin/collections/MutableList.add: R|kotlin/Boolean|>|(String())
+        }
+        ) ?: R|/listOf|<R|kotlin/CharSequence|>()
+    }