The dwo_name was added to dwo files to improve diagnostics in dwp, but
it confuses tools that attempt to load any dwo named by a dwo_name, even
ones inside dwos. Avoid this by keeping track of whether a unit is
already a dwo unit, and if so, not loading further dwos.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267241 91177308-0d34-0410-b5e6-96231b3b80d8