[libc++][test] Fixes using modules in the test suite.

[Mordante]

Summary

Importing modules in the test suite failed due to linker errors. This
fixes the issue and adds a minimal test to validate importing the std
module works.

[philnik]

libcxx/test/std/modules/std.pass.cpp

10

This isn't expected to fail in C++26, right? Could you add a TODO for fixing this?

libcxx/utils/libcxx/test/dsl.py

460

What does the -v do?

[Mordante]

Rebased to trigger CI.

[Mordante]

libcxx/utils/libcxx/test/dsl.py

460

verbose output. In the end I hope all this code can become part of CMake. For now it's easier to debug.

[philnik]

LGTM with TODOs added.

libcxx/utils/libcxx/test/dsl.py

461–463

Please add a TODO to fix this.

libcxx/test/std/modules/std.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTEd: c++03, c++11, c++14, c++17, c++20
+// XFAIL: c++26
+
+// REQUIRES: use_module_std
+
+// A minimal test to validate import works.
+
+import std;
+
+int main(int, char**) {
+  std::println("Hello modular world");
+  return 0;
+}

libcxx/utils/libcxx/test/dsl.py

def applyTo(self, config):
     elif std == 'cxx23':
         std = '23'
     else:
         std = '17' # Not allowed for modules
 
     flags = _getSubstitution('%{flags}', config)
     cmake = _getSubstitution('%{cmake}', config)
 
-    subprocess.check_call([cmake, "-DCMAKE_CXX_STANDARD=" + std, f"-DCMAKE_CXX_FLAGS={flags}", build], env={})
-    subprocess.check_call([cmake, "--build", build], env={})
+    subprocess.check_call([cmake, f"-DCMAKE_CXX_STANDARD={std}", f"-DCMAKE_CXX_FLAGS={flags}", build], env={})
+    subprocess.check_call([cmake, "--build", build, "--", "-v"], env={})
+    config.substitutions = _appendToSubstitution(
+       config.substitutions, "%{link_flags}",  os.path.join(build, "libc++std.a")
+    )
 
   def pretty(self, config, litParams):
     return "building std module with flags {}".format(_getSubstitution('%{flags}', config))
 
 class AddFlagIfSupported(ConfigAction):
     """
     This action adds the given flag to the %{flags} substitution, only if
     the compiler supports the flag.