c++: wrong error with constexpr COMPOUND_EXPR [PR105321]

Here we issue a bogus error for the first assert in the test.  Therein
we have

<retval> = (void) (VIEW_CONVERT_EXPR<bool>(yes) || handle_error ());, VIEW_CONVERT_EXPR<int>(value);

which has a COMPOUND_EXPR, so we get to cxx_eval_constant_expression
<case COMPOUND_EXPR>.  The problem here is that we call

7044             /* Check that the LHS is constant and then discard it.  */
7045             cxx_eval_constant_expression (ctx, op0,
7046                                           true, non_constant_p, overflow_p,
7047                                           jump_target);

where lval is always true, so the PARM_DECL 'yes' is not evaluated into
its value.

Fixed by always passing false for 'lval' in cxx_eval_logical_expression;
there's no case where we actually expect an lvalue from a TRUTH_*.

	PR c++/105321

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_logical_expression): Always pass false for lval
	to cxx_eval_constant_expression.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-105321.C: New test.