Change create_demangled_names_hash to take an objfile_per_bfd_storage

This changes create_demangled_names_hash to take an
objfile_per_bfd_storage parameter.  This makes it clearer where it is
storing the objects it allocates.

gdb/ChangeLog
2019-01-10  Tom Tromey  <tom@tromey.com>

	* symtab.c (create_demangled_names_hash): Change argument to be an
	objfile_per_bfd_storage.
	(symbol_set_names): Update.