When a `trackBy` function is used that accepts a supertype of the iterated
array's type, the loop variable would undesirably be inferred as the supertype
instead of the array's item type. This commit adds an inferred type parameter
to `TrackByFunction` to allow an extra degree of freedom, enabling the
loop value to be inferred as the most narrow type.
Fixes#40125
PR Close#41995
85c7f7691e