Fix reversed output in interpolation concatenation

[Youssef1313]

Fixes #55493
Fixes #55461

Relates to feature/test plan: #51499

[333fred]

Don't use a reverse on the parts arrays, reverse the builder after populating.

[Youssef1313]

@333fred The builder is indeed reversed at the end of the method, but that wasn't enough (we were appending each part in-order, then reversing the final result - but we want to append each part reversed, then reverse the final result).

For the code to be reversing a builder instead of array, I'd have to create a new builder for each BoundInterpolatedString, or loop backwards on Parts and use Add for each part (instead of AddRange(Parts).

Alternatively, attempt to rewrite the code such that we don't need reversing at all (ie, walk from left to right - but I'm not sure how hard is that or if that's possible).

[333fred]

Left to right isn't feasible. If we need to add these in reverse order here as well, we should not use linq to do so.

[333fred]

I'm also concerned about flow analysis. Make sure we have a test like $"{a = null}{a.ToString()}" + $""

[333fred]

@jcouv ptal.

[jcouv]

LGTM Thanks (iteration 4)

[jcouv]

Looks like there is a legitimate failure: InterpolatedStringsAddedUnderObjectAddition3 in the spanish CI branch because of localization of "Object reference not set to an instance of an object.". Consider just printing the exception type name.

[jcouv]

Thanks @Youssef1313