Fix two (three) more issues with unchecked Error.

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. This patch also
includes disabling 2 tests that can't work without assertions enabled,
since llvm_unreachable() with NDEBUG won't crash.

Reviewers: llvm-commits, lhames

Reviewed By: lhames

Subscribers: lhames, pirama

Differential Revision: https://reviews.llvm.org/D36729

llvm-svn: 311739