[docs][llvm-cxxfilt] Write llvm-cxxfilt documentation
There was a stub for llvm-cxxfilt, but it didn't describe the options.
Additionally, it was in markdown, which was causing issues, so as
discussed in https://reviews.llvm.org/D63211, this change replaces the
existing stub with an RST file.
Reviewed by: MaskRay, mattd
Differential Revision: https://reviews.llvm.org/D63722