Fix issue 23367 - Preserve sparse array literal encoding in copyLiteral

[gulugulubing]

This is a follow-up to @ntrel's #23372. It seems that the indexes problem is almost entirely within CTFE (ctfeexpr.d + dinterpret.d), mush less than originally thought.

If it's acceptable, I'll delete #23451.

[thewilsonator]

Have had the chance to run some performance tests? Otherwise looks good.

[gulugulubing]

Have had the chance to run some performance tests? Otherwise looks good.

Just did a test:

enum x = {
    int[10_000_000][] pieces = [];
    pieces.length = 5;
    return pieces[0][0];
}();

void main() {}

The new plan is 6x faster, ~75% less peak memory (RSS ~40% less for this test case).