Alternatively, we could improve the lookup utilities and their usages to
always find the exact override of a symbol from
Collection/Iterable/CharSequence/etc, but since we need to load the
original symbol anyway in cases when the loop subject's type is a type
parameter, we might as well simplify everything and always reference the
original symbol.
Also improve exception message and removed unused declarations in
IrBackendUtils.kt.
Mikhail Glukhikh