c++: Distinguish alignof and __alignof__ in cp_tree_equal [PR97273]
cp_tree_equal currently considers alignof the same as __alignof__, but
these operators are semantically different ever since r8-7957. In the
testcase below, this causes the second static_assert to fail on targets
where alignof(double) != __alignof__(double) because the specialization
table (which uses cp_tree_equal as its equality predicate) conflates the
two dependent specializations integral_constant<__alignof__(T)> and
integral_constant<alignof(T)>.
This patch makes cp_tree_equal distinguish between these two operators
by inspecting the ALIGNOF_EXPR_STD_P flag.