Runtime: Bridging an Error to NSError requires only checking its domain. #7983
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The lazy population of the NSError fields is ordered such that the domain is written into the object last with acq/rel ordering, so another thread only needs to check the domain to see whether the initialization has already happened. (The initialization itself is idempotent, so we can optimistically perform the initialization and discard the results if we race.) Checking the user info is redundant, and is also wrong for real NSError objects, since [NSError errorWithDomain:d code:c userInfo:nil] will in fact plant nil in the userInfo field of the object, leading us to attempt to bridge an already-native NSError.