`c++filt` when given no arguments runs as a REPL, decoding each line as a
decorated name. Unify the test structure to be more uniform, with the tests for
llvm-cxxfilt living under test/tools/llvm-cxxfilt.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286777 91177308-0d34-0410-b5e6-96231b3b80d8
