[IRTranslator] Simplify error handling for translating constants. NFC.
[IRTranslator] Simplify error handling for translating constants. NFC.

We don't need to check whether the fallback path is enabled to return
false. Just do that all the time on error cases, the caller knows (or
at least should know!) how to handle the failing case.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@297535 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit b893e797a57ccfe589eae8b76f442a1c0e3e3fc2)

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