c++: Consider only relevant template arguments in sat_hasher

A large source of cache misses in satisfy_atom is caused by the identity
of an (atom,args) pair within the satisfaction cache being determined by
the entire set of supplied template arguments rather than by the subset
of template arguments that the atom actually depends on.  For instance,
consider

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

The associated constraints of foo and bar are equivalent: they both
consist of the atom range_v<T> (with mapping T -> U).  But the sat_cache
currently will never reuse a satisfaction value between the two atoms
because foo has one template parameter and bar has two, and the
satisfaction cache conservatively assumes that all template parameters
of the constrained decl are relevant to a satisfaction value of one of
its atoms.

This patch eliminates this assumption and makes the sat_cache instead
care about just the subset of args of an (atom,args) pair that is
relevant to satisfaction.

This patch additionally fixes a seemingly latent bug that was found when
testing against range-v3.  In the testcase concepts-decltype2.C below,
during normalization of f's constraints we end up forming a TARGET_EXPR
whose _SLOT has a DECL_CONTEXT that points to g instead of f because
current_function_decl is not updated before we start normalizing.
This patch fixes this accordingly, and also adds a sanity check to
keep_template_parm to verify each found parameter has a valid index.

With this patch, compile time and memory usage for the cmcstl2 test
test/algorithm/set_symmetric_difference4.cpp drops from 8.5s/1.2GB to
3.5s/0.4GB.

gcc/cp/ChangeLog:

	* constraint.cc (norm_info::norm_info): Initialize orig_decl.
	(norm_info::orig_decl): New data member.
	(normalize_atom): When caching an atom for the first time,
	compute a list of template parameters used in the targets of the
	parameter mapping and store it in the TREE_TYPE of the mapping.
	(get_normalized_constraints_from_decl): Set current_function_decl
	appropriately when normalizing.  As an optimization, don't
	set up a push_nested_class_guard when decl has no constraints.
	(sat_hasher::hash): Use this list to hash only the template
	arguments that are relevant to the atom.
	(satisfy_atom): Use this list to compare only the template
	arguments that are relevant to the atom.
	* pt.c (keep_template_parm): Do a sanity check on the parameter's
	index when flag_checking.

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