Fix builtin exception handlers to work across modules

[jagerman]

The builtin exception handler currently doesn't work across modules under clang/libc++ for builtin pybind exceptions like pybind11::error_already_set or pybind11::stop_iteration: under RTLD_LOCAL module loading clang considers each module's exception classes distinct types (this is essentially the same as issue #912). This then means that the base exception translator fails to catch the exceptions and falls through to the generic std::exception handler, which completely breaks things like stop_iteration: only the stop_iteration of the first module loaded actually works properly. Later modules raise a RuntimeError with no message when their iterators try to signal the end.

For example, two modules defined like this exhibit the behaviour under clang++/libc++:

z1.cpp:

#include <pybind11/pybind11.h>
#include <pybind11/stl_bind.h>
namespace py = pybind11;
PYBIND11_MODULE(z1, m) {
    py::bind_vector<std::vector<long>>(m, "IntVector");
}

z2.cpp:

#include <pybind11/pybind11.h>
#include <pybind11/stl_bind.h>
namespace py = pybind11;
PYBIND11_MODULE(z2, m) {
    py::bind_vector<std::vector<double>>(m, "FloatVector");
}

Python:

import z1, z2
for i in z2.FloatVector():
    pass

results in:

Traceback (most recent call last):
  File "zs.py", line 2, in <module>
    for i in z2.FloatVector():
RuntimeError

while if you remove z1 from the import the code runs. (You could also change the order of the import to z2, z1 which lets it run, but would break z1.IntVector's iterator).

This commit fixes the issue by adding a new exception translator each time the internals pointer is initialized from python builtins: this generally means the internals data was initialized by some other module. (The extra translator(s) are skipped under libstdc++).

[dean0x7d]

Looks good to me, but adding a test would be nice. See my comment in #949 about using the new tests module for cross-module testing.

[jagerman]

Closing (this will be merged via #949, which depends on this fix, but also adds the cross module testing apparatus this PR needs).