Convert a few methods to use ErrorOr.

It used to be inconvenient to mix ErrorOr and UniquePtr, but with c++11
they work OK together.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211532 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed