c++: Correct the handling of alignof(expr) [PR88115]

We're currently neglecting to set the ALIGNOF_EXPR_STD_P flag on an
ALIGNOF_EXPR when its operand is an expression.  This leads to us
handling alignof(expr) as if it were written __alignof__(expr), and
returning the preferred alignment instead of the ABI alignment.  In the
testcase below, this causes the first and third static_assert to fail on
x86.

gcc/cp/ChangeLog:

	PR c++/88115
	* cp-tree.h (cxx_sizeof_or_alignof_expr): Add bool parameter.
	* decl.c (fold_sizeof_expr): Pass false to
	cxx_sizeof_or_alignof_expr.
	* parser.c (cp_parser_unary_expression): Pass std_alignof to
	cxx_sizeof_or_alignof_expr.
	* pt.c (tsubst_copy): Pass false to cxx_sizeof_or_alignof_expr.
	(tsubst_copy_and_build): Pass std_alignof to
	cxx_sizeof_or_alignof_expr.
	* typeck.c (cxx_alignof_expr): Add std_alignof bool parameter
	and pass it to cxx_sizeof_or_alignof_type.  Set ALIGNOF_EXPR_STD_P
	appropriately.
	(cxx_sizeof_or_alignof_expr): Add std_alignof bool parameter
	and pass it to cxx_alignof_expr.  Assert op is either
	SIZEOF_EXPR or ALIGNOF_EXPR.

libcc1/ChangeLog:

	PR c++/88115
	* libcp1plugin.cc (plugin_build_unary_expr): Pass true to
	cxx_sizeof_or_alignof_expr.

gcc/testsuite/ChangeLog:

	PR c++/88115
	* g++.dg/cpp0x/alignof6.C: New test.