Make bcache more type-safe

The bcache uses memcpy to make copies of the data passed to it.  In
C++, this is only safe for trivially-copyable types.

This patch changes bcache to require this property, and slightly
changes the API to make it easier to use when copying a single object.
It also makes the new 'insert' template methods return the correct
type.

gdb/gdbtypes.c

@@ -4372,7 +4372,7 @@ check_types_worklist (std::vector<type_equality_entry> *worklist,
 
       /* If the type pair has already been visited, we know it is
 	 ok.  */
-      cache->insert (&entry, sizeof (entry), &added);
+      cache->insert (entry, &added);
       if (!added)
 	continue;