Clean up the logic of the Archive::Child::Child() with an assert to know Err is not...

Clean up the logic of the Archive::Child::Child() with an assert to know Err is not a nullptr
when we are pointed at real data.

David Blaikie pointed out some odd logic in the case the Err value was a nullptr and
Lang Hames suggested it could be cleaned it up with an assert to know that Err is
not a nullptr when we are pointed at real data.  As only in the case of constructing
the sentinel value by pointing it at null data is Err is permitted to be a nullptr,
since no error could occur in that case.

With this change the testing for “if (Err)” is removed from the constructor’s logic
and *Err is used directly without any check after the assert().

llvm-svn: 277776