Switch to consumeError(), since this can crash otherwise.

Summary:
If assertions are disabled, but LLVM_ABI_BREAKING_CHANGES is enabled,
this will cause an issue with an unchecked Success. Switching to
consumeError() is the correct way to bypass the check.

Reviewers: llvm-commits, cfe-commits, arphaman

Reviewed By: arphaman

Subscribers: arphaman, klimek, pirama

Differential Revision: https://reviews.llvm.org/D36728

llvm-svn: 310958