Implement a throwing version of _LIBCPP_ASSERT.
This patch implements changes to allow _LIBCPP_ASSERT to throw on failure
instead of aborting. The main changes needed to do this are:
- Change _LIBCPP_ASSERT to call a handler via a replacable function pointer instead of calling abort directly. Additionally this patch implements two handler functions, one which aborts and another that throws an exception.
- Add _NOEXCEPT_DEBUG macro for disabling noexcept spec on function which contain _LIBCPP_ASSERT. This is required in order to prevent assertion failures throwing through a noexcept function. This macro has no effect unless _LIBCPP_DEBUG_USE_EXCEPTIONS is defined.
Having a non-aborting _LIBCPP_ASSERT is very important to allow sane testing of
debug mode. Currently we can only have one test case per file, since the test
case will cause the program to abort. Testing debug mode this way would require
thousands of test files, most of which would be 95% boiler plate. I don't think
this is a feasible strategy. Fortunately using a throwing debug handler solves
Additionally this patch rewrites the documentation for debug mode.