[FIR] Handle receiver types of extension methods when computing maximally specific types.

Test case (from testPrimitiveReceiver):

fun Short.foo() = 3
fun Int.foo() = 4

1::foo

compiler/testData/diagnostics/tests/callableReference/resolve/overloads.fir.kt

@@ -20,7 +20,7 @@ fun fas() {}
 fun fas(i: Int = 1) {}
 
 fun test() {
-    <!UNRESOLVED_REFERENCE!>B::foo<!> // todo KT-9601 Chose maximally specific function in callable reference
+    B::foo // todo KT-9601 Chose maximally specific function in callable reference
 
     B::bar checkType { _<KFunction1<B, Unit>>() }