Debug info: split out complex DIVariable address expressions into a
separate MDNode so they can be uniqued via folding set magic. To conserve
space, DIVariable nodes are still variable-length, with the last two
fields being optional.

No functional change.
http://reviews.llvm.org/D3526

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212050 91177308-0d34-0410-b5e6-96231b3b80d8