[Clang] - Update code to match upcoming llvm::zlib API.

D28684 changed llvm::zlib to return Error instead of Status.
It was accepted and committed in r292214, but then reverted in r292217
because I missed that clang code also needs to be updated.

Patch do that.

D28684 recommitted again as r292226

Differential revision: https://reviews.llvm.org/D28807

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@292227 91177308-0d34-0410-b5e6-96231b3b80d8