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/test_exceptions.py

@@ -1,6 +1,7 @@
 import pytest
 
 from pybind11_tests import exceptions as m
+import pybind11_cross_module_tests as cm
 
 
 def test_std_exception(msg):
@@ -19,6 +20,27 @@ def test_error_already_set(msg):
     assert msg(excinfo.value) == "foo"
 
 
+def test_cross_module_exceptions():
+    with pytest.raises(RuntimeError) as excinfo:
+        cm.raise_runtime_error()
+    assert str(excinfo.value) == "My runtime error"
+
+    with pytest.raises(ValueError) as excinfo:
+        cm.raise_value_error()
+    assert str(excinfo.value) == "My value error"
+
+    with pytest.raises(ValueError) as excinfo:
+        cm.throw_pybind_value_error()
+    assert str(excinfo.value) == "pybind11 value error"
+
+    with pytest.raises(TypeError) as excinfo:
+        cm.throw_pybind_type_error()
+    assert str(excinfo.value) == "pybind11 type error"
+
+    with pytest.raises(StopIteration) as excinfo:
+        cm.throw_stop_iteration()
+
+
 def test_python_call_in_catch():
     d = {}
     assert m.python_call_in_destructor(d) is True