llvm-undname: More coverage-related cleanups

- The loop in demangleFunctionParameterList() only exits
  on Error, @, and Z. All 3 cases were handled, so the
  rest of the function is DEMANGLE_UNREACHABLE.

- The loop in demangleTemplateParameterList() always returns
  on Error, so there's no need to check for that in the loop
  header and after the loop.

- Add test cases for invalid function parameter manglings.

- Add a (redundant) test case for a simple template parameter
  list mangling.

- Add a test case pointing out that varargs functions aren't
  demangled correctly.

llvm-svn: 362540