This is the FIR fix of the FE change tracked by
https://youtrack.jetbrains.com/issue/KT-45503
Consider the following code:
```
fun foo() {
var l = mutableListOf("")
l += ""
}
```
The above code used to be considered invalid due to ambiguous `+=`. But
after KT-45503, it's now considered valid code because `plus` is not
really a valid option for this code. This is because the return type
of `plus` for `List` is `List`, which does not match the type of
variable `l`.
As for now, FIR rejects the above code due to ambiguous `+=`. This
change fixes that by also consider the return type when resolving `+=`.
Tianyu Geng