Move psymtabs to their own obstack

Previously, the psymtab obstack was just a pointer to the objfile
obstack.  This patch changes psymtabs to use their own obstack,
instead.  A gdb::optional is used to avoid unnecessary allocation when
the obstack is not needed.

After this patch, the psymtab code lifetime model is that, in the core
psymtab code, objects allocated on the psymtab obstack may point to
other such objects, or to objects on the per-BFD obstack -- but never
to the objfile obstack.

Note however that this invariant is only obeyed the core psymtab code,
and even there not quite fully: there is still a link from the psymtab
to the full symtab.

Symbol readers are free to work however they like; and in particular,
even after this patch, in practice all symbol readers violate this
invariant via the read_symtab_private field.

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

	* objfiles.h (objfile::reset_psymtabs): Update.
	* objfiles.c (objfile::objfile): Update.
	* psymtab.h (psymtab_storage::obstack): Update.
	(psymtab_storage::m_obstack): Use gdb::optional.
	(class psymtab_storage): Update comment.  Remove objfile
	parameter.
	* psymtab.c (psymtab_storage::psymtab_storage): Update.