Revert "[libc++] Only include_next C library headers when they exist"

This reverts commit 226409c628.
Breaks check-clang on mac, see comments on https://reviews.llvm.org/D136683

libcxx/include/complex.h

@@ -24,9 +24,13 @@
 #endif
 
 #ifdef __cplusplus
-#  include <ccomplex>
-#elif __has_include_next(<complex.h>)
-#  include_next <complex.h>
-#endif
+
+#include <ccomplex>
+
+#else  // __cplusplus
+
+#include_next <complex.h>
+
+#endif // __cplusplus
 
 #endif // _LIBCPP_COMPLEX_H

libcxx/include/ctype.h

@@ -35,9 +35,7 @@ int toupper(int c);
 #  pragma GCC system_header
 #endif
 
-#if __has_include_next(<ctype.h>)
-#  include_next <ctype.h>
-#endif
+#include_next <ctype.h>
 
 #ifdef __cplusplus
 

libcxx/include/errno.h

@@ -28,9 +28,7 @@ Macros:
 #  pragma GCC system_header
 #endif
 
-#if __has_include_next(<errno.h>)
-#  include_next <errno.h>
-#endif
+#include_next <errno.h>
 
 #ifdef __cplusplus
 

libcxx/include/fenv.h

@@ -56,9 +56,7 @@ int feupdateenv(const fenv_t* envp);
 #  pragma GCC system_header
 #endif
 
-#if __has_include_next(<fenv.h>)
-#  include_next <fenv.h>
-#endif
+#include_next <fenv.h>
 
 #ifdef __cplusplus
 

libcxx/include/float.h

@@ -76,9 +76,7 @@ Macros:
 #  pragma GCC system_header
 #endif
 
-#if __has_include_next(<float.h>)
-#  include_next <float.h>
-#endif
+#include_next <float.h>
 
 #ifdef __cplusplus
 

libcxx/include/inttypes.h

@@ -248,9 +248,7 @@ uintmax_t wcstoumax(const wchar_t* restrict nptr, wchar_t** restrict endptr, int
 #   define __STDC_FORMAT_MACROS
 #endif
 
-#if __has_include_next(<inttypes.h>)
-#  include_next <inttypes.h>
-#endif
+#include_next <inttypes.h>
 
 #ifdef __cplusplus
 

libcxx/include/limits.h

@@ -44,11 +44,7 @@ Macros:
 #endif
 
 #ifndef __GNUC__
-
-#  if __has_include_next(<limits.h>)
-#    include_next <limits.h>
-#  endif
-
+#include_next <limits.h>
 #else
 // GCC header limits.h recursively includes itself through another header called
 // syslimits.h for some reason. This setup breaks down if we directly

libcxx/include/locale.h

@@ -43,8 +43,6 @@ Functions:
 #  pragma GCC system_header
 #endif
 
-#if __has_include_next(<locale.h>)
-#  include_next <locale.h>
-#endif
+#include_next <locale.h>
 
 #endif // _LIBCPP_LOCALE_H

libcxx/include/math.h

@@ -297,9 +297,7 @@ long double    truncl(long double x);
 #  pragma GCC system_header
 #endif
 
-#  if __has_include_next(<math.h>)
-#    include_next <math.h>
-#  endif
+#include_next <math.h>
 
 #ifdef __cplusplus
 

libcxx/include/setjmp.h

@@ -31,9 +31,7 @@ void longjmp(jmp_buf env, int val);
 #  pragma GCC system_header
 #endif
 
-#if __has_include_next(<setjmp.h>)
-#  include_next <setjmp.h>
-#endif
+#include_next <setjmp.h>
 
 #ifdef __cplusplus
 

libcxx/include/stdbool.h

@@ -24,9 +24,7 @@ Macros:
 #  pragma GCC system_header
 #endif
 
-#if __has_include_next(<stdbool.h>)
-#  include_next <stdbool.h>
-#endif
+#include_next <stdbool.h>
 
 #ifdef __cplusplus
 #undef bool

libcxx/include/stddef.h

@@ -42,9 +42,7 @@ Types:
 #  pragma GCC system_header
 #endif
 
-#  if __has_include_next(<stddef.h>)
-#    include_next <stddef.h>
-#  endif
+#include_next <stddef.h>
 
 #ifdef __cplusplus
     typedef decltype(nullptr) nullptr_t;

libcxx/include/stdint.h

@@ -120,8 +120,6 @@ Macros:
 #   define __STDC_CONSTANT_MACROS
 #endif
 
-#if __has_include_next(<stdint.h>)
-#  include_next <stdint.h>
-#endif
+#include_next <stdint.h>
 
 #endif // _LIBCPP_STDINT_H

libcxx/include/stdio.h

@@ -104,9 +104,7 @@ void perror(const char* s);
 #  pragma GCC system_header
 #endif
 
-#  if __has_include_next(<stdio.h>)
-#    include_next <stdio.h>
-#  endif
+#include_next <stdio.h>
 
 #ifdef __cplusplus
 

libcxx/include/stdlib.h

@@ -90,9 +90,7 @@ void *aligned_alloc(size_t alignment, size_t size);                       // C11
 #  pragma GCC system_header
 #endif
 
-#  if __has_include_next(<stdlib.h>)
-#    include_next <stdlib.h>
-#  endif
+#include_next <stdlib.h>
 
 #ifdef __cplusplus
 extern "C++" {

libcxx/include/string.h

@@ -57,9 +57,7 @@ size_t strlen(const char* s);
 #  pragma GCC system_header
 #endif
 
-#if __has_include_next(<string.h>)
-#  include_next <string.h>
-#endif
+#include_next <string.h>
 
 // MSVCRT, GNU libc and its derivates may already have the correct prototype in
 // <string.h>. This macro can be defined by users if their C library provides

libcxx/include/tgmath.h

@@ -24,11 +24,13 @@
 #endif
 
 #ifdef __cplusplus
-#  include <ctgmath>
-#else
-#  if __has_include_next(<tgmath.h>)
-#    include_next <tgmath.h>
-#  endif
-#endif
+
+#include <ctgmath>
+
+#else  // __cplusplus
+
+#include_next <tgmath.h>
+
+#endif // __cplusplus
 
 #endif // _LIBCPP_TGMATH_H

libcxx/include/wchar.h

@@ -120,9 +120,7 @@ size_t wcsrtombs(char* restrict dst, const wchar_t** restrict src, size_t len,
 #define __CORRECT_ISO_CPP_WCHAR_H_PROTO
 #endif
 
-#  if __has_include_next(<wchar.h>)
-#    include_next <wchar.h>
-#  endif
+#include_next <wchar.h>
 
 // Determine whether we have const-correct overloads for wcschr and friends.
 #if defined(_WCHAR_H_CPLUSPLUS_98_CONFORMANCE_)