llvm-cxxfilt: support the `-s` option
llvm-cxxfilt: support the `-s` option

This is a stub implementation of the `-s` or `--format` option that
allows the user to specify the demangling style.  Since we only support
the Itanium (GNU) style demangling, auto is synonymous with `gnu`.
Simply swallow the option to permit some level of commandline
compatibility.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@292706 91177308-0d34-0410-b5e6-96231b3b80d8

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