[llvm] [llvm-nm] Introduce synthetic flag (PR #138232)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

I finally found some time to catch up on old reviews that have been sat idle.

Please could you add test cases that exercise the error paths. Also a test case that shows that when --synthetic is used on a non-ELF object, no error is produced.

https://github.com/llvm/llvm-project/pull/138232