This requires making template argument extraction a bit more robust

GCC 5.4.1 created debug info only reports the first argument for
boost::variant<int, bool>:

    #include <boost/variant/variant.hpp
    int main() { boost::variant<int, float> v = 1; return 0; }

    py print(gdb.parse_and_eval('v').type)

      -> boost::variant<int, float>

    py print(gdb.parse_and_eval('v').type.template_argument(0))

      -> int

    py print(gdb.parse_and_eval('v').type.template_argument(1))

       -> Traceback (most recent call last):
          File \"<string>\", line 1, in <module>
          RuntimeError: No argument 1 in template.
          Error while executing Python code.

Change-Id: Iedca8b073078c93449ab61bb2cab05d6cd9803ba
Reviewed-by: Christian Stenger <christian.stenger@qt.io>