Partially revert r152770.  That commit moved the default handlers to their own file.  But it also did some refactoring.  It is the latter that is being reverted.  The refactoring had accidentally removed the required effect that the default unexpected_handler calls std::terminate(), which is a visible effect.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@174532 91177308-0d34-0410-b5e6-96231b3b80d8