Use flags enum for cooked_index_entry::full_name

Use flags enum for cooked_index_entry::full_name

I found a small bug coming from a couple of  recent patches of mine for
cooked_index_entry::full_name.

First, commit aab26529b30 (Add "Ada linkage" mode to
cooked_index_entry::full_name) added a small hack to optionally
compute the Ada linkage name.

Then, commit aab2ac34d7f (Avoid excessive CU expansion on failed
matches) changed the relevant expand_symtabs_matching implementation
to use this feature.

However, the feature was used unconditionally, causing a bad side
effect: the non-canonical name is now used for all languages, not just
Ada.  But, for C++ this is wrong.

Furthermore, consider the declaration of full_name:

   const char *full_name (struct obstack *storage,
			 bool for_main = false,
			 bool for_ada_linkage = false,
 			 const char *default_sep = nullptr) const;

... and then consider this call in cooked_index::dump:

       gdb_printf ("    qualified:  %s\n",
		  entry->full_name (&temp_storage, false, "::"));

Oops!  The "::" is silently converted to 'true' here.

To fix both of these problems, this patch changes full_name to accept
a flags enum rather than booleans.  This avoids the type-safety
problem.

Then, full_name is changed to remove the "Ada" flag when the entry is
not in fact an Ada symbol.

Regression tested on x86-64 Fedora 40.

Approved-By: Simon Marchi <simon.marchi@efficios.com>