llvm-undname: Improve string literal demangling with embedded \0 chars

- Don't assert when a string looks like a u32 string to the heuristic
  but doesn't have a length that's 0 mod 4.  Instead, classify those
  as u16 with embedded \0 chars. Found by oss-fuzz.
- Print embedded nul bytes as \0 instead of \x00.

llvm-svn: 358835