Use new and delete for psymtabs

Use new and delete for psymtabs

This changes psymtabs to be allocated with new and destroyed with
delete.  As a consequence, the psymtab free-list is also removed.

The motivation for this is to let symbol readers subclass
partial_symtab.

gdb/ChangeLog
2020-01-26  Tom Tromey  <tom@tromey.com>

	* mdebugread.c (parse_partial_symbols): Use discard_psymtab.
	* psymtab.h (class psymtab_storage) <free_psymtabs>: Remove.
	* psymtab.c (psymtab_storage): Delete psymtabs.
	(psymtab_storage::allocate_psymtab): Use new.
	(psymtab_storage::discard_psymtab): Use delete.
	* psympriv.h (struct partial_symtab): Add constructor and
	initializers.

Change-Id: I4e78ac538fc0ea52b57489c1afb8f935a30941ef