restrict map_rect to legal data-only args

The following program should not compile because xi[i] isn't an entirely data-only expression. The value xi is a data variable, but the index i is local. That allows illegal programs like this to get compiled:

functions {
  vector f(vector beta, vector theta, data real[] xr, data int[] xi ) {
    return [1.0]';
  }
}
transformed data {
  vector[2] beta;
  vector[2] theta[2];
  real xr[2, 2];
  int xi[2, 2, 2];
}
parameters {
  real y;
}
transformed parameters {
  real s = 0;
  for (i in 1:2) {
    // BAD!  DATA CHANGING BETWEEN CALLS
    s += sum(map_rect(f, beta, theta, xr, xi[i]));
  }
}

These will produce different results with MPI turned on and turned off, which we don't ever want to happen.

Start from the compiler's map_rect argument validation and reproduce the Stan program shown in the issue. Check that an indexed expression such as xi[i] is rejected as not entirely data-only, and verify that the invalid program no longer compiles; the issue does not name specific files or tests.