Import libcxxrt master 698997bfde1fc81b0bc0bfd1a1e5e3ad6f33b634

Interesting fixes:
045c52c Mark cxa_allocate_exception, cxa_free_exception and

__cxa_init_primary_exception noexcept.

8a2f123 Define _LIBCXXRT_NOEXCEPT in cxxabi.h and use it instead of

throw()

9529236 Fix memory corruption in cpp_demangle_read_sname()
8f5c74e Add test cases, fix more bugs, and improve perf
391a3dc Add a simple implementation of __cxa_call_terminate
40e4fa2 mark std::terminate as noreturn and noexcept
5eede09 Print diagnostics in default std::terminate handler