[WSL] It should be possible to chain comma operators

[Robin Morisset]

int f() {
    int x;
    x = 42;
    x *= 2, x += 1, x *= 3;
    return x;
}
fails with a parser error, yet works if either of the ',' operator are replaced by a ';'.
It should be accepted, there is no reason for chained comma operators to be refused.

[Myles C. Maxfield]

Migrated to https://github.com/gpuweb/WHLSL/issues/119