[llvm-lto] Uniform error handling. NFC.
[llvm-lto] Uniform error handling. NFC.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@266255 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit a6ab26162c685d7b31583e33b182d71a966acee4)

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