We currently call the `llvm_unreachable` for the following YAML:
```
--- !ELF
FileHeader:
Class: ELFCLASS32
Data: ELFDATA2LSB
Type: ET_REL
Machine: EM_NONE
Flags: [ ]
```
it happens because the `Flags` key is present, though `EM_NONE` is a
machine type that has no known `EF_*` values and we call `llvm_unreachable` by mistake.
Differential revision: https://reviews.llvm.org/D86138