Add include guard
Add include guard

Summary: Add missing include guard LLVM_DEMANGLE_DEMANGLE_H in llvm/Demangle/Demangle.h

Reviewers: erik.pilkington

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50042

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@338694 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