[AutoDiff] Add VJP for 'Array.init(repeating:count:)'. (#27255)
Add a VJP for `Array.init(repeating:count:)` so that it can be differentiated. The VJP is merely a transpose since `init(repeating:count:)` is linear w.r.t. the repeated value. Its tranpose is a linear map that takes an `Array.TangentVector` and returns the sum of all of its elements.
Resolves [TF-802](https://bugs.swift.org/browse/TF-802).
