__cxa_demangle: allow demangling invocation blocks

The block invocation function uses an extension where the prefix is ___Z
as opposed to _Z.  This should make the tests pass again.

Disable a negative test which was testing a crasher.  The symbol being
demangled is not a valid mangled symbol and will return a nullptr.

Adjust the type info decoding test to be a valid symbol name.

llvm-svn: 286793