[libcxx] Implement locale.h to fix modules build

Summary:
Because `locale.h` isn't part of the libc++ modules the class definitions it provides are exported as part of `__locale` (since it happens to be build first). This breaks `<clocale>` which exports `std::lconv` without including `<__locale>`.

This patch implements `locale.h` to fix this issue, it also adds support for testing libc++ with modules.




Reviewers: mclow.lists, rsmith, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26826

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@287413 91177308-0d34-0410-b5e6-96231b3b80d8

include/locale.h

@@ -0,0 +1,45 @@
+// -*- C++ -*-
+//===---------------------------- locale.h --------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_LOCALE_H
+#define _LIBCPP_LOCALE_H
+
+/*
+    locale.h synopsis
+
+Macros:
+
+    LC_ALL
+    LC_COLLATE
+    LC_CTYPE
+    LC_MONETARY
+    LC_NUMERIC
+    LC_TIME
+
+Types:
+
+    lconv
+
+Functions:
+
+   setlocale
+   localeconv
+
+*/
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#include_next <locale.h>
+
+#endif  // _LIBCPP_LOCALE_H