message: [mlir] Fix tests after Chris implemented string escaping in MLIR
  We don't need to C-escape any more, so don't. Also, change the expected escaping syntax to be the slightly noisier version that LLVM emits.
diff: @@ -113,7 +113,7 @@ std::string Token::getStringValue() const {
  break;
  }
  - assert(i + 1 < e && "invalid string should be caught by lexer");
  + assert(i + 1 <= e && "invalid string should be caught by lexer");
  auto c2 = bytes[i++];
  assert(llvm::isHexDigit(c1) && llvm::isHexDigit(c2) && "invalid escape");
  