Revert "Update aosp/master libcxx rebase to r263688"

The world is burning.

This reverts commit c004fd909c, reversing
changes made to 1418e4163d.

test/std/strings/basic.string/string.cons/default_noexcept.pass.cpp

@@ -7,8 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++98, c++03
-
 // <string>
 
 // basic_string()
@@ -31,6 +29,7 @@ struct some_alloc
 
 int main()
 {
+#if __has_feature(cxx_noexcept)
     {
         typedef std::string C;
         static_assert(std::is_nothrow_default_constructible<C>::value, "");
@@ -43,4 +42,5 @@ int main()
         typedef std::basic_string<char, std::char_traits<char>, some_alloc<char>> C;
         static_assert(!std::is_nothrow_default_constructible<C>::value, "");
     }
+#endif
 }