Fix esoteric regression in the previous commit

Implement the check for duplicate SWIG_derives_from<> specializations which
could happen if a template type with default parameters was exported both with
implicit default values used for these parameters and the same values
specified explicitly.

This makes the code more complicated and is very rarely needed (in addition to
the above, the template class must also have a base class to trigger the bug),
but it does happen and used to work, so restore this after breaking it in the
last commit and reenable the template_default2 unit test.

Examples/test-suite/c/Makefile.in

@@ -95,7 +95,6 @@ FAILING_CPP_TESTS := \
     special_variables \
     template_arg_typename \
     template_basic \
-    template_default2 \
     template_explicit \
     template_ns4 \
     template_tbase_template \