Add missing error handling in llvm-lto
Add missing error handling in llvm-lto

Annoyingly, ErrorOr allows to *not check* the error when things go
well. It will crash badly when there is an error though. It should
runtime assert when it is used without being checked!

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@263577 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm