[clang] remove isDefaulted bit from TemplateArgument

The IsDefaulted bit being part of a canonical TemplateArgument
doesn't make sense, as that information is not information a
canonical type should have.

In C++, all template specialization types for ther same template
are the same if the full list of template arguments is the same,
an argument being defaulted or not doesn't matter.

Moreover, this information is already available in the sugared
template specialization type, in the sense that, taking the as-written
list and matching it up to the template parameters, any parameters which
are left without a corresponding template argument must have been defaulted.

This patch besides removing that bit, changes the current DebugInfo users to derive
that information from the as-written argument list. And it goes a little beyond
that by wiring up the actual sugared TemplateArguments, so the Debug Info
produced is also richer.

This patch is a performance improvement, as the TemplateArgument is one of the
hottest data structures for C++ compilation:

The small regression on `-O0 -g` test is explained by the increased amount of
debug info generated.