Add flag to allow using kotlin.Result as a return type

#KT-26659 Fixed

compiler/testData/diagnostics/testsWithStdLib/coroutines/allowResultInReturnTypeWithFlag.kt

@@ -0,0 +1,8 @@
+// !ALLOW_RESULT_RETURN_TYPE
+
+fun result(): Result<Int> = TODO()
+val resultP: Result<Int> = result()
+
+fun f(r1: Result<Int>?) {
+    r1 <!RESULT_CLASS_WITH_NULLABLE_OPERATOR!>?:<!> <!UNUSED_EXPRESSION!>0<!>
+}

compiler/testData/diagnostics/testsWithStdLib/coroutines/allowResultInReturnTypeWithFlag.txt

@@ -0,0 +1,5 @@
+package
+
+public val resultP: kotlin.Result<kotlin.Int>
+public fun f(/*0*/ r1: kotlin.Result<kotlin.Int>?): kotlin.Unit
+public fun result(): kotlin.Result<kotlin.Int>