llvm-cxxfilt: support `-t` to demangle types

llvm-cxxfilt: support `-t` to demangle types

By default c++filt demangles functions, though you can optionally pass
`-t` to have it decode types as well, behaving nearly identical to
`__cxa_demangle`.  Add support for this mode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292576 91177308-0d34-0410-b5e6-96231b3b80d8