llvm-cxxfilt: filter out invalid manglings

c++filt does not attempt to demangle symbols which do not match its
expected format.  This means that the symbol must start with _Z or ___Z
(block invocation function extension).  Any other symbols are returned
as is.  Note that this is different from the behaviour of __cxa_demangle
which will demangle fragments.

llvm-svn: 292467