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

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.

Differential Revision: https://reviews.llvm.org/D138512

Author: ldionne

libcxx/include/cassert

@@ -18,7 +18,14 @@ Macros:
 
 #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

libcxx/include/csignal

@@ -42,8 +42,12 @@ int raise(int sig);
 #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)

libcxx/include/cstdarg

@@ -33,7 +33,14 @@ Types:
 
 #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

libcxx/include/ctime

@@ -47,7 +47,14 @@ int timespec_get( struct timespec *ts, int base); // C++17
 
 #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