c++: find_template_parameters and PARM_DECLs [PR105797]

As explained in r11-4959-gde6f64f9556ae3, the atom cache assumes two
equivalent expressions (according to cp_tree_equal) must use the same
template parameters (according to find_template_parameters).  This
assumption turned out to not hold for TARGET_EXPR, which was addressed
by that commit.

But this assumption apparently doesn't hold for PARM_DECL either:
find_template_parameters walks its DECL_CONTEXT but cp_tree_equal by
default doesn't consider DECL_CONTEXT unless comparing_specializations
is set.  Thus in the first testcase below, the atomic constraints of #1
and #2 are equivalent according to cp_tree_equal, but according to
find_template_parameters the former uses T and the latter uses both T
and U (surprisingly).

We could fix this assumption violation by setting comparing_specializations
in the atom_hasher, which would make cp_tree_equal return false for the
two atoms, but that seems overly pessimistic here.  Ideally the atoms
should continue being considered equivalent and we instead fix
find_template_paremeters to return just T for #2's atom.

To that end this patch makes for_each_template_parm_r stop walking the
DECL_CONTEXT of a PARM_DECL.  This should be safe to do because
tsubst_copy / tsubst_decl only substitutes the TREE_TYPE of a PARM_DECL
and doesn't bother substituting the DECL_CONTEXT, thus the only relevant
template parameters are those used in its type.  any_template_parm_r is
currently responsible for walking its TREE_TYPE, but I suppose it now makes
sense for for_each_template_parm_r to do so instead.

In passing this patch also makes for_each_template_parm_r stop walking
the DECL_CONTEXT of a VAR_/FUNCTION_DECL since doing so after walking
DECL_TI_ARGS is redundant, I think.

I experimented with not walking DECL_CONTEXT for CONST_DECL, but the
second testcase below demonstrates it's necessary to walk it.

	PR c++/105797

gcc/cp/ChangeLog:

	* pt.cc (for_each_template_parm_r) <case FUNCTION_DECL, VAR_DECL>:
	Don't walk DECL_CONTEXT.
	<case PARM_DECL>: Likewise.  Walk TREE_TYPE.
	<case CONST_DECL>: Simplify.
	(any_template_parm_r) <case PARM_DECL>: Don't walk TREE_TYPE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-decltype4.C: New test.
	* g++.dg/cpp2a/concepts-memfun3.C: New test.

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