Hack to allow concat to work even when an Array isn't assignable to ReadonlyArray

[ghost]

Fixes #20268
Uses T[] | ReadonlyArray<T> instead of just ReadonlyArray<T>. There are probably a bunch of other places where we would need to do this to fix this error in general, but better to wait on #20454 for those instead.

[RyanCavanaugh]

Have you run this on RWC?

[ghost]

Will do tomorrow. This makes the signature more lenient (assuming covariant arrays) so I wouldn't expect any new RWC failures.

[ghost]

There are some rwc failures on master, but I compared this branch to master and didn't see any differences.

[ahejlsberg]

@mhegazy @sandersn This PR appears to account for 50% of the slowdown we've seen in the checker between 2.6.2 and 2.7. It single-handedly slows down the check phase of compiling the compiler by about 7%. I haven't researched exactly why, but I suspect it has to do with us performing lots of structural comparisons of array types.

The ideal way to fix this issue would be to have all the callback function types in Array<T> provide a ReadonlyArray<T> (modifying the array in a callback is surely a bad thing), but obviously that would be a breaking change.

[mhegazy]

@ahejlsberg has a possible fir to reorder the #21462, can you take a look.