[libc++] Add missing __has_include checks for C headers not provided by libc++

[ldionne]

Summary

This makes the library consistent in how it handles C library headers. For C headers provided by libc++,
we unconditionally include <foo.h> from <cfoo>, and then <foo.h> conditionally include_next <foo.h>.
For headers not provided by libc++, <cfoo> conditionally includes the system's <foo.h> directly.

[philnik]

LGTM with green CI.

libcxx/include/cassert

25

Could we add something like

#ifdef _LIBCPP_ASSERT_H
#  error "Does libc++ provide an assert.h now?"
#endif

to make sure?

[ldionne]

Address comment.

[ldionne]

The AIX failures are flakes.

libcxx/include/cassert

 Macros:
 
     assert
 
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
-#include <assert.h>
+
+// <assert.h> is not provided by libc++
+#if __has_include(<assert.h>)
+#  include <assert.h>
+#  ifdef _LIBCPP_ASSERT_H
+#    error "If libc++ starts defining <assert.h>, the __has_include check should move to libc++'s <assert.h>"
+#  endif
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif

libcxx/include/csignal

 
 }  // std
 
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 
+// <signal.h> is not provided by libc++
 #if __has_include(<signal.h>)
-# include <signal.h>
+#  include <signal.h>
+#  ifdef _LIBCPP_SIGNAL_H
+#    error "If libc++ starts defining <signal.h>, the __has_include check should move to libc++'s <signal.h>"
+#  endif
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::sig_atomic_t _LIBCPP_USING_IF_EXISTS;
 using ::signal _LIBCPP_USING_IF_EXISTS;
 using ::raise _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSIGNAL

libcxx/include/cstdarg

Types:
     va_list
 
 }  // std
 
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
-#include <stdarg.h>
+
+// <stdarg.h> is not provided by libc++
+#if __has_include(<stdarg.h>)
+#  include <stdarg.h>
+#  ifdef _LIBCPP_STDARG_H
+#    error "If libc++ starts defining <stdarg.h>, the __has_include check should move to libc++'s <stdarg.h>"
+#  endif
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::va_list _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSTDARG

libcxx/include/ctime

size_t strftime(char* restrict s, size_t maxsize, const char* restrict format,
                 const tm* restrict timeptr);
 int timespec_get( struct timespec *ts, int base); // C++17
 }  // std
 
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
-#include <time.h>
+
+// <time.h> is not provided by libc++
+#if __has_include(<time.h>)
+#  include <time.h>
+#  ifdef _LIBCPP_TIME_H
+#    error "If libc++ starts defining <time.h>, the __has_include check should move to libc++'s <time.h>"
+#  endif
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::clock_t _LIBCPP_USING_IF_EXISTS;