Make psymtab_storage::free_psymtabs private

This adds a new psymtab allocation method to psymtab_storage and
changes the free_psymtabs member to be private.  While not strictly
necessary, this seems like a decent cleanup, and also makes it simpler
to move psymtabs off of obstacks entirely, should that prove
desirable.

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

	* psymtab.h (psymtab_storage::allocate_psymtab): New method.
	<free_psymtabs>: Now private.
	* psymtab.c (psymtab_storage::allocate_psymtab): Implement.
	(allocate_psymtab): Use new method.