libstdc++: Use Clang attribute instead of __constinit

Clang doesn't support the __constinit extension that we use pre-C++20,
but it does have its own equivalent attribute that can be used instead.

This makes it a little easier to use Clang to build libstdc++ (which
isn't supported. but is sometimes attempted for esoteric targets).

libstdc++-v3/ChangeLog:

	* src/c++11/cxx11-ios_failure.cc (__constinit): Define as
	equivalent attribute for Clang.
	* src/c++11/future.cc (__constinit): Likewise.
	* src/c++11/system_error.cc (__constinit): Likewise.
	* src/c++17/memory_resource.cc (__constinit): Likewise.

libstdc++-v3/src/c++11/cxx11-ios_failure.cc

@@ -42,6 +42,10 @@
 # error This file should not be compiled for this configuration.
 #endif
 
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
 namespace
 {
   struct io_error_category final : std::error_category

libstdc++-v3/src/c++11/future.cc

@@ -25,6 +25,10 @@
 #include <future>
 #include <bits/functexcept.h>
 
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
 namespace
 {
   struct future_error_category final : public std::error_category

libstdc++-v3/src/c++11/system_error.cc

@@ -37,6 +37,10 @@
 #include <windows.h>
 #endif
 
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
 namespace
 {
   using std::string;

libstdc++-v3/src/c++17/memory_resource.cc

@@ -32,6 +32,10 @@
 # include <bits/move.h>		// std::__exchange
 #endif
 
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION