[ObjectYAML] Remove redundant .str().c_str() (NFC) (#167154)
We can drop .str().c_str() here because all of the following are of
type StringRef:
- E.Name
- the second parameter of llvm::yaml::IO::enumCase
- the second parameter of llvm::yaml::IO::bitSetCase
Identified with readability-redundant-string-cstr.