c++: Distinguish alignof and __alignof__ in cp_tree_equal [PR97273]

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.

gcc/cp/ChangeLog:

	PR c++/88115
	PR libstdc++/97273
	* tree.c (cp_tree_equal) <case ALIGNOF_EXPR>: Return false if
	ALIGNOF_EXPR_STD_P differ.

gcc/testsuite/ChangeLog:

	PR c++/88115
	PR libstdc++/97273
	* g++.dg/template/alignof3.C: New test.

(cherry picked from commit 592fe221735bdaa375b1834dd49ce125d0b600d8)