c++: Use two levels of caching in satisfy_atom

This improves the effectiveness of caching in satisfy_atom by querying
the cache again after we've instantiated the atom's parameter mapping.

Before instantiating its mapping, the identity of an (atom,args) pair
within the satisfaction cache is determined by idiosyncratic things like
the level and index of each template parameter used in targets of the
parameter mapping.  For example, the associated constraints of foo in

  template <class T> concept range = range_v<T>;
  template <class U, class V> void foo () requires range<U> && range<V>;

are range_v<T> (with mapping T -> U) /\ range_v<T> (with mapping T -> V).
If during satisfaction the template arguments supplied for U and V are
the same, then the satisfaction value of these two atoms will be the
same (despite their uninstantiated parameter mappings being different).

But sat_cache doesn't see this because it compares the uninstantiated
parameter mapping and the supplied template arguments of sat_entry's
independently.  So satisy_atom on this latter atom will end up fully
evaluating it instead of reusing the satisfaction value of the former.

But there is a point when the two atoms do look the same to sat_cache,
and that's after instantiating their parameter mappings.  By querying
the cache again at this point, we can avoid substituting the same
instantiated parameter mapping into the same expression a second time
around.

With this patch, compile time and memory usage for the cmcstl2 test
test/algorithm/set_symmetric_diference4.cpp drops from 11s/1.4GB to
8.5s/1.2GB with an --enable-checking=release compiler.

gcc/cp/ChangeLog:

	* cp-tree.h (ATOMIC_CONSTR_MAP_INSTANTIATED_P): Define this flag
	for ATOMIC_CONSTRs.
	* constraint.cc (sat_hasher::hash): Use hash_atomic_constraint
	if the flag is set, otherwise keep using a pointer hash.
	(sat_hasher::equal): Return false if the flag's setting differs
	on two atoms.  Call atomic_constraints_identical_p if the flag
	is set, otherwise keep using a pointer equality test.
	(satisfy_atom): After instantiating the parameter mapping, form
	another ATOMIC_CONSTR using the instantiated mapping and query
	the cache again.  Cache the satisfaction value of both atoms.
	(diagnose_atomic_constraint): Simplify now that the supplied
	atom has an instantiated mapping.