Quoted identifiers are missing quotes in output dzn

[jmjrawlings]

Given the test suite model unit/output/record_access_printing.mzn:

record(var 1..2: x, var 1..2: 'x+'): r;
any: a :: output = r.x;
any: b :: output = r.'x+';
solve minimize a + b;

solving this yields:

r = (x: 1, x+: 1);
a = 1;
b = 1;
_objective = 2;

The missing quotes around the x+ identifier mean that in this case the output dzn cannot be parsed by the minizinc parser