terminate, unexpected and new handlers. If terminating while an exception is...

terminate, unexpected and new handlers. If terminating while an exception is unwinding, an attempt is made to print out the what() string if the exception is derived from std::exception.  __terminate(handler) and __unexpected(handler) helpers are present in anticipation of other parts of libc++abi needing to call these interfaces with custom handlers.

llvm-svn: 145948