c++: Fix another PCH hash_map issue [PR96901]

c++: Fix another PCH hash_map issue [PR96901]

The recent libstdc++ changes caused lots of libstdc++-v3 tests FAILs
on i686-linux, all of them in the same spot during constexpr evaluation
of a recursive _S_gcd call.
The problem is yet another hash_map that used the default hasing of
tree keys through pointer hashing which is preserved across PCH write/read.
During PCH handling, the addresses of GC objects are changed, which means
that the hash values of the keys in such hash tables change without those
hash tables being rehashed.  Which in the fundef_copies_table case usually
means we just don't find a copy of a FUNCTION_DECL body for recursive uses
and start from scratch.  But when the hash table keeps growing, the "dead"
elements in the hash table can sometimes reappear and break things.
In particular what I saw under the debugger is when the fundef_copies_table
hash map has been used on the outer _S_gcd call, it didn't find an entry for
it, so returned a slot with *slot == NULL, which is treated as that the
function itself is used directly (i.e. no recursion), but that addition of
a hash table slot caused the recursive _S_gcd call to actually find
something in the hash table, unfortunately not the new *slot == NULL spot,
but a different one from the pre-PCH streaming which contained the returned
toplevel (non-recursive) call entry for it, which means that for the
recursive _S_gcd call we actually used the same trees as for the outer ones
rather than a copy of those, which breaks constexpr evaluation.

2020-09-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/96901
	* tree.h (struct decl_tree_traits): New type.
	(decl_tree_map): New typedef.

	* constexpr.c (fundef_copies_table): Change type from
	hash_map<tree, tree> * to decl_tree_map *.