c++/contracts: contract pack expansion referring to parameter [PR125645]

Contract conditions are parsed outside the function body, so
`at_function_scope_p()' is false when their parameter pack expansions
are formed.  However, function parameter packs in contract conditions
should use local specializations during substitution, just as they do
within a function body.

local_bindings_p seems like a better test than at_function_scope_p, but we
need to move sk_contract earlier in scope_kind for it to give the right
answer.

	PR c++/125645

gcc/cp/ChangeLog:

	* cp-tree.h (PACK_INDEX_PARENTHESIZED_P): Fix incorrect
	documentation in usage.
	* name-lookup.h (enum scope_kind): Move sk_contract before
	sk_function_parms.
	* pt.cc (make_pack_expansion): Use local_bindings_p.

gcc/testsuite/ChangeLog:

	* g++.dg/contracts/cpp26/fold-pr125645.C: New test.

Signed-off-by: Wang Jinghao <zheng.xianyuwang@gmail.com>
Co-authored-by: Jason Merrill <jason@redhat.com>