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/debug/debug_throw.pass.cpp

@@ -11,6 +11,9 @@
 // UNSUPPORTED: libcpp-no-exceptions
 // MODULES_DEFINES: _LIBCPP_DEBUG=0
 
+// Can't test the system lib because this test enables debug mode
+// UNSUPPORTED: with_system_cxx_lib
+
 // Test that the default debug handler can be overridden and test the
 // throwing debug handler.