Make getCompileUnit non-const and return the current DIE if it
happens to be a compile unit. Noticed on inspection and tested
via calling on a newly created compile unit. No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181835 91177308-0d34-0410-b5e6-96231b3b80d8