Add tests on coercion to Unit for callable references

#KT-11723

compiler/testData/codegen/box/callableReference/bound/coercionToUnit.kt

@@ -0,0 +1,20 @@
+// !LANGUAGE: +NewInference
+// IGNORE_BACKEND: JS_IR, JVM_IR
+// WITH_RUNTIME
+
+class Foo
+
+class Builder {
+    var size: Int = 0
+
+    fun addFoo(foo: Foo): Builder {
+        size++
+        return this
+    }
+}
+
+fun box(): String {
+    val b = Builder()
+    listOf(Foo(), Foo(), Foo()).forEach(b::addFoo)
+    return if (b.size == 3) "OK" else "Fail"
+}