[ELF] Convert ELF.h to Expected<T>.
This has two advantages:
1) We slowly move away from ErrorOr to the new handling interface,
in the hope of having an uniform error handling in LLVM, eventually.
2) We're starting to have *meaningful* error messages for invalid
object ELF files, rather than a generic "parse error". At some point
we should include also the offset to improve the quality of the
diagnostic.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287081
91177308-0d34-0410-b5e6-
96231b3b80d8
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