Make extract_symbols.py be compatible with Python 3
Make extract_symbols.py be compatible with Python 3

This involved running 2to3 on it and adjusting all uses of subprocess to use
universal_newlines=True so the output is text instead of binary. It remains
compatible with Python 2.7.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@274365 91177308-0d34-0410-b5e6-96231b3b80d8

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