[libc++][modules] Improves export validation.

[Mordante]

Summary

Adds tests to guard against the issue solved in D156592.

Depends on D156592

[ChuanqiXu]

LGTM. Thanks.

libcxx/test/libcxx/module_std.gen.py

print(
     f" --load=%{{test-tools}}/clang_tidy_checks/libcxx-tidy.plugin "
     "  -- %{flags} %{compile_flags} "
     "  | sort > %t.module"
 )
 
 
 # Compare the sum of the parts with the main module.
 print(f"// RUN{BLOCKLIT}: diff -u %t.all_partitions %t.module")
+
+# Basic smoke test. Import a module and try to compile when using all
+# exported names. This validates the clang-tidy script does not accidentally
+# add named declarations to the list that are not available.
+print(f"// RUN{BLOCKLIT}: echo 'import std;' > %t.compile.pass.cpp")
+print(f"// RUN{BLOCKLIT}: cat %t.all_partitions >> %t.compile.pass.cpp")
+print(f"// RUN{BLOCKLIT}: %{{cxx}} %{{flags}} %{{compile_flags}} -fsyntax-only %t.compile.pass.cpp")

libcxx/test/tools/clang_tidy_checks/header_exportable_declarations.cpp

if (is_reserved_name(name))
       return;
 
     // For modules (std, std.compat) only take the declarations exported from the partitions.
     // Making sure no declatations of headers are compared.
     if (file_type_ == FileType::Module)
       if (clang::Module* M = decl->getOwningModule(); M && M->Kind != clang::Module::ModulePartitionInterface)
         return;
 
+    // For module partitions only take exported declarations.
+    if (file_type_ == FileType::ModulePartition)
+      if (decl->getModuleOwnershipKind() != clang::Decl::ModuleOwnershipKind::VisibleWhenImported)
+        return;
+
     if (decls_.contains(name)) {
       // For modules avoid exporting the same named declaration twice. For
       // header files this is common and valid.
       if (file_type_ == FileType::ModulePartition)
         // After the warning the script continues.
         // The test will fail since modules have duplicated entries and headers not.
         llvm::errs() << "Duplicated export of '" << name << "'.\n";
       else