s/std::set/DenseSet/; NFC
s/std::set/DenseSet/; NFC

We only use this set for `insert` and `count`, so a hashing container
seems better here.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@340783 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm