Revert "Revert "Merge to upstream r304942.""

This reverts commit 38a0d5af7e.

Test: make checkbuild
Test: ./run_tests.py
Test: ./run_tests.py --bitness 64

test/libcxx/experimental/containers/sequences/dynarray/dynarray.mutate/default.pass.cpp

@@ -8,6 +8,8 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11
+// XFAIL: availability
+
 // dynarray.data
 
 // void fill(const T& v);