The function convention for the first argument is not identified as indirect-out.
This lets alias analysis assume that the memory pointed to by argument 0 cannot be written by the called function.
The problem is that subscripting a LazyFilterCollection (with the base index, e.g. `Int`) does not work as expected, because it returns the nth element of the base collection!
The fix is to implement the subscript "manually".
Fixes a mis-compile.
rdar://152160748
7eb123ff29