Despite it's name, GetTemplateArgument was only really working for Type
template arguments. This adds the ability to retrieve integral arguments
as well (which I've needed for the std::bitset data formatter).
I've done this by splitting the function into three pieces. The idea is
that one first calls GetTemplateArgumentKind (first function) to
determine the what kind of a parameter this is. Based on that, one can
then use specialized functions to retrieve the correct value. Currently,
I only implement two of these: GetTypeTemplateArgument and