DIBuilder: simplify array generation to produce true zero-length arrays

For some anachronistic reason we were producing {i32 0} for zero-length
debug info arrays.

(this change is paired with a Clang change and may cause temporary
buildbot noise)

Let's not.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200721 91177308-0d34-0410-b5e6-96231b3b80d8