Add missing noexcept keyword to default constructor of each
_Optional_payload specialization.

	PR libstdc++/85642 fix is_nothrow_default_constructible<optional<T>>
	* include/std/optional (_Optional_payload): Add noexcept to default
	constructor. Re-indent.
	(_Optional_payload<_Tp, true, true, true>): Likewise. Add noexcept to
	constructor for copying disengaged payloads.
	(_Optional_payload<_Tp, true, false, true>): Likewise.
	(_Optional_payload<_Tp, true, true, false>): Likewise.
	(_Optional_payload<_Tp, true, false, false>): Likewise.
	* testsuite/20_util/optional/cons/85642.cc: New.
	* testsuite/20_util/optional/cons/value_neg.cc: Adjust dg-error lines.

From-SVN: r259928