llvm-undname: Several behavior-preserving changes to increase coverage

- Replace `Error = true` in a few branches that are truly unreachable
  with DEMANGLE_UNREACHABLE

- Remove early return early in startsWithLocalScopePattern() because
  it's redundant with the next two early returns

- Remove unreachable `case '0'` (it's handled in the branch below)

- Remove an unused bool return

- Add test coverage for several early error returns, mostly in
  array type parsing

llvm-svn: 362506