Revert "[ELF] Remove redundant isExported computation"
This reverts commit 1a4d6de1b532149b10522eae5dabce39e5f7c687.
It causes problems with `-static-pie`. See repro at
https://github.com/llvm/llvm-project/commit/1a4d6de1b532149b10522eae5dabce39e5f7c687#commitcomment-151827775
