c++: ICE with constexpr call that returns a PMF [PR98551]

c++: ICE with constexpr call that returns a PMF [PR98551]

We shouldn't do replace_result_decl after evaluating a call that returns
a PMF because PMF temporaries aren't wrapped in a TARGET_EXPR (and so we
can't trust ctx->object), and PMF initializers can't be self-referential
anyway, so replace_result_decl would always be a no-op.

To that end, this patch changes the relevant AGGREGATE_TYPE_P test to
CLASS_TYPE_P, which should rule out PMFs (as well as arrays, which we
can't return and therefore won't see here).  This fixes an ICE from the
sanity check in replace_result_decl in the below testcase during
constexpr evaluation of the call f() in the initializer g(f()).

gcc/cp/ChangeLog:

	PR c++/98551
	* constexpr.c (cxx_eval_call_expression): Check CLASS_TYPE_P
	instead of AGGREGATE_TYPE_P before calling replace_result_decl.

gcc/testsuite/ChangeLog:

	PR c++/98551
	* g++.dg/cpp0x/constexpr-pmf2.C: New test.

(cherry picked from commit bb1f0b50abbfa01e0ed720a5225a11aa7af32a89)