[Hashing] Forward-declare hash_value for tuples.
[Hashing] Forward-declare hash_value for tuples.

This allows hash_combine to work on tuples containing no types within the
LLVM namespace, because two-phase name lookup is weird.

(cherry picked from commit 691f840fa4ddd6b47462e2769da0fb9d4be63c77)

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