[ELF] Reorder BitcodeFiles.empty() to call thinLTOCreateEmptyIndexFiles() in only one place

It makes the --plugin-opt=obj-path= and --plugin-opt=thinlto-index-only=
behavior more consistent - the files will be created in the
BitcodeFiles.empty() case, but I assume whether it behaves this way is
not required by anyone.

LTOObj->run() cannot run with empty BitcodeFiles. There would be an error:

    ld.lld: error: No available targets are compatible with triple ""

Differential Revision: https://reviews.llvm.org/D61635

llvm-svn: 360129