Don't bother creating LabelBegin for .dwo units

This would help catch cases where we might otherwise try to reference a
dwo CU label, which would be weird - because without relocations in the
dwo file it's not generally meaningful to talk about the CU offsets
there (or, if it is, we can do so in absolute terms without using a
relocation to compute it).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221078 91177308-0d34-0410-b5e6-96231b3b80d8
Be sure to initialize DwarfCompileUnit::LabelBegin now that it may be skipped in initSection

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221079 91177308-0d34-0410-b5e6-96231b3b80d8