Remove DwarfUnit::LabelEnd in favor of computing the length of the section directly

This was a compile-unit specific label (unused in type units) and seems
unnecessary anyway when we can more easily directly compute the size of
the compile unit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221067 91177308-0d34-0410-b5e6-96231b3b80d8