c++: Fix production comment for mangle_ctor_vtbl_for_type.

mangle.cc::mangle_ctor_vtbl_for_type has

   write_string ("_Z");
   write_string ("TC");

and hence the function comment should read

   We use the production
   <special-name> ::= TC <type> ...

instead of the current

   <special-name> ::= CT <type> ...

gcc/cp/
	* mangle.cc (mangle_ctor_vtbl_for_type): Fix production in comment.