Remove safeguard from RoundTripYAML pass.

RoundTripYAML pass is removed from the regular execution pass in r204296,
so the safeguard to protect it from OOM error is no longer needed, because
we are sure that the pass is only used for tests, and test files are all
small.

We also want to see RoundTripYAML pass to fail in tests if it fails,
rather than silently skipping failing tests.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@204772 91177308-0d34-0410-b5e6-96231b3b80d8