Use htab_up for demangled hash

Use htab_up for demangled hash

This changes objfile_per_bfd_storage::demangled_names_hash to be an
htab_up.  This lets us remove some manual management code from the
objfile_per_bfd_storage destructor.

gdb/ChangeLog
2019-03-15  Tom Tromey  <tom@tromey.com>

	* symtab.c (create_demangled_names_hash): Update.
	(symbol_set_names): Update.
	* objfiles.h (struct objfile_per_bfd_storage)
	<demangled_names_hash>: Now an htab_up.
	* objfiles.c (objfile_per_bfd_storage): Simplify.