llvm-dwp: Support empty .dwo files

Though a bit odd, this is handy for a few reasons - for example, in a
build system that wants consistent input/output of build steps, but
where split-dwarf might be overriden/disabled by the user on a per-file
basis.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261987 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed