Revert "DIBuilder: Remove dead code"

This reverts commit r218820.  It turns out that Adrian has an
outstanding SROA patch that uses this.

I've updated it to forward to `createExpression()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218828 91177308-0d34-0410-b5e6-96231b3b80d8
DIBuilder: Remove duplicated comments, NFC

These comments already appear in the header, and some of them are
out-of-date anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218829 91177308-0d34-0410-b5e6-96231b3b80d8