Move the 'store' method to index_cache_store_context

I think it is cleaner for 'store' to be a method on
index_cache_store_context rather than on the global index cache
itself.  This patch makes this change.

gdb/dwarf2/cooked-index.c

@@ -842,7 +842,7 @@ cooked_index::maybe_write_index (const index_cache_store_context &ctx)
   if (index_for_writing () != nullptr)
     {
       /* (maybe) store an index in the cache.  */
-      global_index_cache.store (ctx);
+      ctx.store ();
     }
   m_state->set (cooked_state::CACHE_DONE);
 }