copiler: add function attributes to some runtime calls

This allows better escape analysis even without being able to see the
entire program. This makes the stack allocation test case more complete
but probably won't have much of an effect outside of that (as the
compiler is able to infer these attributes in the whole-program
functionattrs pass).

transform/testdata/allocs2.go

@@ -22,6 +22,13 @@ func main() {
 
 	s4 := make([]byte, 300) // OUT: object allocated on the heap: object size 300 exceeds maximum stack allocation size 256
 	readByteSlice(s4)
+
+	s5 := make([]int, 4) // OUT: object allocated on the heap: escapes at line 27
+	s5 = append(s5, 5)
+
+	s6 := make([]int, 3)
+	s7 := []int{1, 2, 3}
+	copySlice(s6, s7)
 }
 
 func derefInt(x *int) int {
@@ -45,3 +52,7 @@ func returnIntSlice(s []int) []int {
 }
 
 func getUnknownNumber() int
+
+func copySlice(out, in []int) {
+	copy(out, in)
+}