[nfc][cxx-interop] Add a basic test for using std::string.

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

@@ -1,3 +1,9 @@
 module StdVector {
   header "std-vector.h"
-}
+  requires cplusplus
+}
+
+module StdString {
+  header "std-string.h"
+  requires cplusplus
+}

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

@@ -0,0 +1,8 @@
+#ifndef TEST_INTEROP_CXX_STDLIB_INPUTS_STD_STRING_H
+#define TEST_INTEROP_CXX_STDLIB_INPUTS_STD_STRING_H
+
+#include <string>
+
+using CxxString = std::string;
+
+#endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_STRING_H

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

@@ -0,0 +1,29 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-cxx-interop)
+//
+// REQUIRES: executable_test
+//
+// Enable this everywhere once we have a solution for modularizing libstdc++: rdar://87654514
+// REQUIRES: OS=macosx
+
+import StdlibUnittest
+import StdString
+import std.string
+
+var StdStringTestSuite = TestSuite("StdString")
+
+StdStringTestSuite.test("init") {
+    let s = CxxString()
+    expectEqual(s.size(), 0)
+    expectTrue(s.empty())
+}
+
+// LLVM module verification fails for calls to std::string::push_back: rdar://88343327
+// StdStringTestSuite.test("push back") {
+//     var s = CxxString()
+//     s.push_back(42)
+//     expectEqual(s.size(), 1)
+//     expectFalse(s.empty())
+//     expectEqual(s[0], 42)
+// }
+
+runAllTests()