support debugger in HasMethods::add_method

When using the perl debugger, it is useful to keep track of the correct
file name and line numbers per method.  Package::Stash allows for this
in add_symbol (wrapped by MOP method add_package_symbol), but requires
that additional arguments are passed in to track this information.  This
changelist modifies add_method so that, if the debugger is enabled, we
pass the additional parameters into add_package_symbol so as not to
clobber the debugger info in %DB::sub.