Clang has two different ways it emits array constants (from InitListExprs and from APValues), and both had some ability to emit zeroinitializer, but neither was able to catch all cases where we could use zeroinitializer reliably. In particular, emitting from an APValue would fail to notice if all the explicit array elements happened to be zero. In addition, for large arrays where only an initial portion has an explicit initializer, we would emit the complete initializer (which could be huge) rather than emitting only the non-zero portion. With this change, when the element would have a suffix of more than 8 zero elements, we emit the array constant as a packed struct of its initial portion followed by a zeroinitializer constant for the trailing zero portion.
In passing, I found a bug where SemaInit would sometimes walk the entire array when checking an initializer that only covers the first few elements; that's fixed here to unblock testing of the rest.