c++: discarded-value and constexpr

I've been thinking for a while that the 'lval' parameter needed a third
value for discarded-value expressions; most importantly,
cxx_eval_store_expression does extra work for an lvalue result, and we also
don't want to do the l->r conversion.

Mostly this is pretty mechanical.  Apart from the _store_ fix, I also use
vc_discard for substatements of a STATEMENT_LIST other than a stmt-expr
result, and avoid building _REFs to be ignored in a few other places.

gcc/cp/ChangeLog:

	* constexpr.cc (enum value_cat): New. Change all 'lval' parameters
	from int to value_cat.  Change most false to vc_prvalue, most true
	to vc_glvalue, cases where the return value is ignored to
	vc_discard.
	(cxx_eval_statement_list): Only vc_prvalue for stmt-expr result.
	(cxx_eval_store_expression): Only build _REF for vc_glvalue.
	(cxx_eval_array_reference, cxx_eval_component_reference)
	(cxx_eval_indirect_ref, cxx_eval_constant_expression): Likewise.

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