[BinaryFormat] Remove dangling declaration of DiscriminantString
[BinaryFormat] Remove dangling declaration of DiscriminantString

The implementation of the function was deleted in r324426. This also
removes the declaration.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@324474 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 97d47c0facf330da35e695cea877eea77926c3a9)

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