[libc++] Use init_priority(100) when possible

Priorities below 101 are reserved for the implementation, so that's what
we should be using here. That is unfortunately only supported on more
recent versions of Clang. See https://reviews.llvm.org/D31413 for details.

Differential Revision: https://reviews.llvm.org/D95972

Author: ldionne

libcxx/include/__config

@@ -1330,7 +1330,13 @@ extern "C" _LIBCPP_FUNC_VIS void __sanitizer_annotate_contiguous_container(
 #endif
 
 #if __has_attribute(init_priority)
-# define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
+    // TODO: Remove this once we drop support for building libc++ with old Clangs
+#   if (defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER < 1200) || \
+       (defined(__apple_build_version__) && __apple_build_version__ < 13000000)
+#     define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
+#   else
+#     define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(100)))
+#   endif
 #else
 # define _LIBCPP_INIT_PRIORITY_MAX
 #endif

libcxx/src/experimental/memory_resource.cpp

@@ -76,7 +76,9 @@ union ResourceInitHelper {
   ~ResourceInitHelper() {}
 };
 
+# 79 "memory_resource.cpp" 1 3
 _LIBCPP_SAFE_STATIC ResourceInitHelper res_init _LIBCPP_INIT_PRIORITY_MAX;
+# 81 "memory_resource.cpp" 2
 
 } // end namespace
 

libcxx/src/iostream.cpp

@@ -73,7 +73,12 @@ __asm__("?wclog@" _LIBCPP_ABI_NAMESPACE_STR "@std@@3V?$basic_ostream@_WU?$char_t
 #endif
 ;
 
+// Hacky way to make the compiler believe that we're inside a system header so
+// it doesn't flag the use of the init_priority attribute with a value that's
+// reserved for the implementation (we're the implementation).
+# 80 "iostream.cpp" 1 3
 _LIBCPP_HIDDEN ios_base::Init __start_std_streams _LIBCPP_INIT_PRIORITY_MAX;
+# 82 "iostream.cpp" 2
 
 // On Windows the TLS storage for locales needs to be initialized before we create
 // the standard streams, otherwise it may not be alive during program termination