Sink dwarf unit length emission down into DwarfUnit::emitHeader

This allows the CU label to be emitted only for compile units, as
they're the only ones that need it (so they can be referenced from
pubnames)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221072 91177308-0d34-0410-b5e6-96231b3b80d8