Fix memory allocations in ipa-modref.

Fix memory allocations in ipa-modref.

Pair ggc_delete with ggc_alloc_no_dtor.  I copy same scheme as used by Martin
in ipa-fnsummary, that is creating a static member function create_ggc hidding
the ugly bits and using it in ipa-modref.c.

I also noticed that modref-tree leaks memory on destruction/collapse method and
fixed that.

Bootstrapped/regtested x86_64-linux.

gcc/ChangeLog:

2020-09-24  Jan Hubicka  <hubicka@ucw.cz>

	* ipa-modref-tree.h (modref_base::collapse): Release memory.
	(modref_tree::create_ggc): New member function.
	(modref_tree::colapse): Release memory.
	(modref_tree::~modref_tree): New destructor.
	* ipa-modref.c (modref_summaries::create_ggc): New function.
	(analyze_function): Use create_ggc.
	(modref_summaries::duplicate): Likewise.
	(read_modref_records): Likewise.
	(modref_read): Likewise.