This test:

#include <iostream>
#include <stdexcept>
#include <exception>
#include <cstdlib>

void handler1()
{
    std::cout << "handler 1\n";
    std::abort();
}

void handler2()
{
    std::cout << "handler 2\n";
    std::abort();
}

struct A
{
    ~A() {std::set_terminate(handler2);}
};

void f2()
{
    std::set_terminate(handler1);
    throw std::runtime_error("throw in f2");
}

void f1() noexcept
{
    A a;
    f2();
}

int main()
{
    f1();
}

Should output:

handler 1

When run against the trunk libc++abi.   Note that the trunk libc++abi won't run with the trunk libc++, but this will change soon (within a few days).

When the noexcept is violated, clang currently outputs a call to std::terminate().  Just prior to that call clang needs to call __cxa_begin_catch(unwind_arg) to make this work.

Reference [except.handle]/p7:

Also, an implicit handler is considered active when std::terminate() or std::unexpected() is entered due to a throw.

With the Itanium ABI, the way you make a handler active is by calling __cxa_begin_catch(unwind_arg).