Add tests on coercion to Unit for callable references

#KT-11723

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

@@ -0,0 +1,16 @@
+// !LANGUAGE: +NewInference
+// IGNORE_BACKEND: JVM_IR
+
+fun foo(s: String): Boolean {
+    if (s != "kotlin") throw AssertionError(s)
+    return true
+}
+
+fun bar(f: (String) -> Unit) {
+    f("kotlin")
+}
+
+fun box(): String {
+    bar(::foo)
+    return "OK"
+}