[FIR2IR] Fix translating primitive array types

This commit fixes two issues in the existing implementation of translating primitive array types:

 * IrType.getArrayElementType throws an exception when the receiver is a primitive array type, because IR expects primitive array types use symbols defined in IrBuiltIns, but fir2ir translation doesn't;
 * IteratorNext.toCallable assumes all element types are boxed.

The first issue is fixed by changing the fir2ir type translation to use symbols in IrBuiltIns for primitive array types, and the second by not unboxing primitive types.

compiler/testData/codegen/box/controlStructures/forInArray/forPrimitiveIntArray.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun box() : String {
     val a = IntArray (5)
     var i = 0