[libc++][C++03] Use __cxx03/ headers in C++03 mode

Author: philnik777

.github/workflows/libcxx-build-and-test.yaml

@@ -43,6 +43,7 @@ jobs:
       fail-fast: false
       matrix:
         config: [
+          'frozen-cxx03-headers',
           'generic-cxx03',
           'generic-cxx26',
           'generic-modules'

libcxx/cmake/caches/frozen-cxx03-headers.cmake

@@ -0,0 +1,2 @@
+set(LIBCXX_TEST_PARAMS "std=c++03;test_frozen_cxx03_headers=True" CACHE STRING "")
+set(LIBCXXABI_TEST_PARAMS "${LIBCXX_TEST_PARAMS}" CACHE STRING "")

libcxx/include/__config

@@ -152,11 +152,6 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #  define _LIBCPP_TOSTRING2(x) #x
 #  define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
 
-// NOLINTNEXTLINE(libcpp-cpp-version-check)
-#  if __cplusplus < 201103L
-#    define _LIBCPP_CXX03_LANG
-#  endif
-
 #  ifndef __has_constexpr_builtin
 #    define __has_constexpr_builtin(x) 0
 #  endif

libcxx/include/__configuration/language.h

@@ -18,6 +18,10 @@
 
 // NOLINTBEGIN(libcpp-cpp-version-check)
 #ifdef __cplusplus
+#  if __cplusplus < 201103L
+#    define _LIBCPP_CXX03_LANG
+#  endif
+
 #  if __cplusplus <= 201103L
 #    define _LIBCPP_STD_VER 11
 #  elif __cplusplus <= 201402L

libcxx/include/__cxx03/__config

@@ -157,11 +157,6 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #  define _LIBCPP_TOSTRING2(x) #x
 #  define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
 
-// NOLINTNEXTLINE(libcpp-cpp-version-check)
-#  if __cplusplus < 201103L
-#    define _LIBCPP_CXX03_LANG
-#  endif
-
 #  ifndef __has_constexpr_builtin
 #    define __has_constexpr_builtin(x) 0
 #  endif

libcxx/include/__flat_map/sorted_unique.h

@@ -11,10 +11,6 @@
 
 #include <__config>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
-
 #if _LIBCPP_STD_VER >= 23
 
 _LIBCPP_BEGIN_NAMESPACE_STD