llvm-undname: Yet more coverage for error paths

- For error returns in demangleSpecialTableNode(),
  demangleLocalStaticGuard(), RTTITypeDescriptor,
  demangleRttiBaseClassDescriptorNode(), demangleUnsigned(),
  demangleUntypedVariable() (via RttiBaseClassArray)

- For ?_A and ?_P which are handled at early levels of the
  demangler but are not implemented in a later stage; this
  is now more obvious

- Replace a "default:" with an explicit list of cases, to
  get -Wswitch check we list all cases

llvm-svn: 362520