Report boxed math warning on function that boxes primitive return value

With the new :warn-on-boxed ( CLJ-1325 Closed ), these examples do not report a boxed math warning although they each do boxing:

user=> (defn f1 [^long x] (inc x))
f1
user=> (defn f2 [x] (aget (long-array [1 2]) 0))
f2
user=> (defn f3 [x] (aget (int-array [1 2]) 0))
f3
user=> (defn f4 [^String s] (.indexOf s "a"))

Cause: emitBoxReturn has a hard-coded call to box a prim return value.

Solution: If *unchecked-math* is set to :warn-on-boxed, emit a warning on boxing of primitive numeric return types.

Patch: