[libcxx][modules] Fix <stddef.h>'s module definition

[EricWF]

Summary

Currently libc++'s stddef.h is treated as a textual header by the module map, and technically this is correct because the header is non-modular (at least during normal compilation).
Unfortunatly this means that the stddef module doesn't own any of the definitions it should, like size_t and ptrdiff_t. which end up getting owned by random STL headers.

Fortunately I believe that libc++'s stddef.h *is* modular due to Clang internals which handle modules containing stddef.h differently. Specifically Clang translates the module definition module stddef_h { header "stddef.h" export *} into

module stddef_h {
  textual header "clang/builtin-includes/.../stddef.h"
  header "libcxx/include/stddef.h"
}

This means that the __need_* macros will always be resolved and #undef-ed by Clang's stddef.h header before processing libc++'s. Therefore we should be able to treat libc++'s header as non-textual. This allows libc++ to actually build a stddef_h module containing the correct definitions.

I've tested this change both with and without local sub-module visibility and on both Linux and OS X.

@rsmith What do you think? Does the above rational make sense?

include/module.modulemap

module setjmp_h {
       header "setjmp.h"
       export *
     }
     // FIXME: <stdalign.h> is missing.
     // <signal.h> provided by C library.
     // <stdarg.h> provided by compiler.
     // <stdbool.h> provided by compiler.
     module stddef_h {
-      // <stddef.h>'s __need_* macros require textual inclusion.
-      textual header "stddef.h"
+      // NOTE: <stddef.h> should technically be a textual header due to the
+      // __need_* macros. However Clang transforms this module definition into:
+      //   module stddef_h {
+      //      textual header "clang/builtin-includes/stddef.h"
+      //      header "libcxx/include/stddef.h"
+      //  }
+      // Therefore Clang's stddef.h resolves the __need_* macros before
+      // building libc++'s header as a non-textual module.
+      header "stddef.h"
     }
     module stdint_h {
       header "stdint.h"
       export *
       // FIXME: This module only exists on OS X and for some reason the
       // wildcard above doesn't export it.
       export Darwin.C.stdint
     }

test/libcxx/modules/cstddef_h_exports.sh.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-support
+
+// Test that stddef.h provides size_t
+
+// RUN: %build_module
+
+#include <cstddef>
+
+int main() {
+  size_t x; ((void)x);
+  std::size_t y; ((void)y);
+}

test/libcxx/modules/stddef_h_exports.sh.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-support, verify-support
+
+// Test that stddef.h provides size_t
+
+// RUN: %compile_module %verify
+
+#include <ciso646>
+
+int main() {
+  size_t x; // expected-error {{missing '#include <stddef.h>'}}
+}

test/libcxx/modules/stdlib_h_exports.sh.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-support, verify-support
+
+// Test that stddef.h provides size_t
+
+// RUN: %build_module
+
+#include <stdlib.h>
+
+int main() {
+  size_t x;
+}