The function "warning" returns bool to indicated whether or not any
diagnostic was actually emitted; warn_deprecated_use should as well.

It's also unnecessary to duplicate the warning code between the cases
of null or non-null "decl", since the actual warnings were the same.
The only thing that's different is whether we indicate the source
location of "decl".

Tested x86_64-pc-linux-gnu.  OK for trunk?