Editorial: Avoid hitting assertion in GetUTCEpochNanoseconds

GetUTCEpochNanoseconds has an assertion that MakeDate, MakeTime, and
MakeDay all result in a finite value. However, several places call
GetUTCEpochNanoseconds with a year value that may be excessively large,
notably from parsing an ISO string such as +999999-01-01, which would
cause that assertion to be hit.

Add checks in the appropriate places to make sure that the year value is
not excessively large.

Closes: #2729

spec/instant.html

@@ -523,6 +523,7 @@ <h1>ToTemporalInstant ( _item_ )</h1>
         1. If _item_ is not a String, throw a *TypeError* exception.
         1. Let _parsed_ be ? ParseTemporalInstantString(_item_).
         1. If _parsed_.[[Z]] is *true*, let _offsetNanoseconds_ be 0; otherwise, let _offsetNanoseconds_ be ! ParseDateTimeUTCOffset(_parsed_.[[OffsetString]]).
+        1. If abs(ISODateToEpochDays(_parsed_.[[Year]], _parsed_.[[Month]] - 1, _parsed_.[[Day]])) > 10<sup>8</sup>, throw a *RangeError* exception.
         1. Let _epochNanoseconds_ be GetUTCEpochNanoseconds(_parsed_.[[Year]], _parsed_.[[Month]], _parsed_.[[Day]], _parsed_.[[Hour]], _parsed_.[[Minute]], _parsed_.[[Second]], _parsed_.[[Millisecond]], _parsed_.[[Microsecond]], _parsed_.[[Nanosecond]], _offsetNanoseconds_).
         1. If IsValidEpochNanoseconds(_epochNanoseconds_) is *false*, throw a *RangeError* exception.
         1. Return ! CreateTemporalInstant(_epochNanoseconds_).

spec/plaindatetime.html

@@ -941,6 +941,7 @@ <h1>
       </emu-note>
       <emu-alg>
         1. Assert: IsValidISODate(_year_, _month_, _day_) is *true*.
+        1. If abs(ISODateToEpochDays(_year_, _month_ - 1, _day_)) > 10<sup>8</sup> + 1, return *false*.
         1. Let _ns_ be ℝ(GetUTCEpochNanoseconds(_year_, _month_, _day_, _hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_)).
         1. If _ns_ &le; nsMinInstant - nsPerDay, then
           1. Return *false*.