DIEEntry: Refer to the specified DIE via reference rather than pointer.

Makes some more cases (the unit tests, specifically), lexically
compatible with a change to unique_ptr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207261 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed