Tweak to mangling spec

[Jason Merrill]

The mangling spec currently has

   <template-args> ::= I <template-arg>+ E

but we've gotten a bug report 
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40808) that requires 
mangling of a dependent template-id with no template args; the above 
should be changed to

   <template-args> ::= I <template-arg>* E

Jason