Turn on hashing by default for split dwarf compile units.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203680 91177308-0d34-0410-b5e6-96231b3b80d8
Use values we've already computed, update comment.
No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203681 91177308-0d34-0410-b5e6-96231b3b80d8