C++: Fix a FN in cpp/cleartext-storage-file - #23019

The cpp/cleartext-storage-file query (and its friend cpp/cleartext-transmission) blocks flow to all integral types in the name of reducing false positives. However, since char is an integral type this also blocks common code such as:

std::string copy_characters(std::string s)
{
  std::string result;
  for (char ch : s) result += ch;
  return result;
}
...
std::string s = copy_characters("sensitive stuff");
fprintf(log, "%s", s.c_str());

since the flow is blocked when it reaches the ch variable. This PR excludes char-like types from this restriction.

As this PR is preparatory for a much larger PR I also included a small related drive-by fix which is necessary for the larger PR. That commit changes the flow so that we select *x (i.e., the indirection of x) as the flow source instead of x (when x is a variable of reference type). You can see the effect on the subsequent commit which accepts the changes to the path.