FIR: Replicate coercion-to-unit behavior from FE 1.0

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

@@ -0,0 +1,18 @@
+FILE: elvisAndUnit.kt
+    public final fun foo(x: R|(kotlin/Int) -> kotlin/Unit|): R|kotlin/Unit| {
+    }
+    public final class A : R|kotlin/Any| {
+        public constructor(): R|A| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final fun bar(): R|kotlin/Unit| {
+        }
+
+    }
+    public final fun main(a: R|A?|, y: R|kotlin/String|): R|kotlin/Unit| {
+        R|/foo|(<L> = foo@fun <anonymous>(it: R|kotlin/Int|): R|kotlin/Unit| <inline=NoInline>  {
+            R|<local>/a|?.{ $subj$.R|/A.bar|() } ?: R|<local>/y|.R|kotlin/String.get|(Int(0))
+        }
+        )
+    }

compiler/fir/analysis-tests/testData/resolve/inference/elvisAndUnit.kt

@@ -0,0 +1,11 @@
+fun foo(x: (Int) -> Unit) {}
+
+class A {
+    fun bar() {}
+}
+
+fun main(a: A?, y: String) {
+    foo {
+        a?.bar() ?: y.get(0)
+    }
+}