Fix builtin exception handlers to work across modules

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 then means that the base exception
translator fails to catch the exceptions and the fall 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 trying to invoke their iterators.

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

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++).

tests/pybind11_cross_module_tests.cpp

@@ -17,4 +17,11 @@ PYBIND11_MODULE(pybind11_cross_module_tests, m) {
     // Definitions here are tested by importing both this module and the
     // relevant pybind11_tests submodule from a test_whatever.py
 
+    // test_exceptions.py
+    m.def("raise_runtime_error", []() { PyErr_SetString(PyExc_RuntimeError, "My runtime error"); throw py::error_already_set(); });
+    m.def("raise_value_error", []() { PyErr_SetString(PyExc_ValueError, "My value error"); throw py::error_already_set(); });
+    m.def("throw_pybind_value_error", []() { throw py::value_error("pybind11 value error"); });
+    m.def("throw_pybind_type_error", []() { throw py::type_error("pybind11 type error"); });
+    m.def("throw_stop_iteration", []() { throw py::stop_iteration(); });
+
 }