[cxx-interop] Initial tests for std::function usage

This makes sure that `std::function` is imported consistently on supported platforms, and that it allows basic usage: calling a function with `callAsFunction`, initializing an empty function, and passing a function retrieved from C++ back to C++ as a parameter.

rdar://103979602

Author: egorzhdan

test/Interop/Cxx/stdlib/Inputs/module.modulemap

@@ -33,3 +33,8 @@ module StdUniquePtr {
   header "std-unique-ptr.h"
   requires cplusplus
 }
+
+module StdFunction {
+  header "std-function.h"
+  requires cplusplus
+}

test/Interop/Cxx/stdlib/Inputs/std-function.h

@@ -0,0 +1,16 @@
+#ifndef TEST_INTEROP_CXX_STDLIB_INPUTS_STD_FUNCTION_H
+#define TEST_INTEROP_CXX_STDLIB_INPUTS_STD_FUNCTION_H
+
+#include <functional>
+
+using FunctionIntToInt = std::function<int(int)>;
+
+FunctionIntToInt getIdentityFunction() {
+  return [](int x) { return x; };
+}
+
+bool isEmptyFunction(FunctionIntToInt f) { return !(bool)f; }
+
+int invokeFunction(FunctionIntToInt f, int x) { return f(x); }
+
+#endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_FUNCTION_H

test/Interop/Cxx/stdlib/use-std-function.swift

@@ -0,0 +1,29 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop)
+// RUN: %target-run-simple-swift(-I %S/Inputs -cxx-interoperability-mode=upcoming-swift)
+
+// REQUIRES: executable_test
+
+import StdlibUnittest
+import StdFunction
+
+var StdFunctionTestSuite = TestSuite("StdFunction")
+
+StdFunctionTestSuite.test("init empty") {
+  let f = FunctionIntToInt()
+  expectTrue(isEmptyFunction(f))
+
+  let copied = f
+  expectTrue(isEmptyFunction(copied))
+}
+
+StdFunctionTestSuite.test("call") {
+  let f = getIdentityFunction()
+  expectEqual(123, f(123))
+}
+
+StdFunctionTestSuite.test("retrieve and pass back as parameter") {
+  let res = invokeFunction(getIdentityFunction(), 456)
+  expectEqual(456, res)
+}
+
+runAllTests()