[libc++] Implement terminate(), unexpected() and uncaught_exceptions() on Windows

Summary:
This patch implements the following functions on Windows by forwarding to the MSVCRT:

* `get_terminate()`
* `set_terminate()`
* `terminate()`
* `set_unexpected()`
* `get_unexpected()`
* `unexpected()`
* `uncaught_exception()`

* `uncaught_exceptions()`

Reviewers: compnerd, rnk, majnemer, smeenai

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28442

llvm-svn: 291343