[llvm-readobj, libSupport] - Refine the implementation of the code that dumps build attributes.

This implementation of `ELFDumper<ELFT>::printAttributes()` in llvm-readobj has issues:
1) It crashes when the content of the attribute section is empty.
2) It uses `unwrapOrError` and `reportWarning` calls, though
   ideally we want to use `reportUniqueWarning`.
3) It contains a TODO about redundant format version check.

`lib/Support/ELFAttributeParser.cpp` uses a hardcoded constant instead of the named constant.

This patch fixes all these issues.

Differential revision: https://reviews.llvm.org/D92318