c++: Fix return deduction of lambda in discarded stmt.

A return statement in a discarded statement is not used for return type
deduction, but we still want to do deduction for a return statement in a
lambda in a discarded statement.

gcc/cp/ChangeLog
2020-03-02  Jason Merrill  <jason@redhat.com>

	PR c++/93442
	* parser.c (cp_parser_lambda_expression): Clear in_discarded_stmt.