[XRay][clang] Use llvm::to_string instead of std::string
[XRay][clang] Use llvm::to_string instead of std::string

This should unbreak some bots.

Follow-up on D30388.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@299044 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang