FIR: strengthen resolution success check for augmented array set call

This commit removes some false ambiguities &
fixes compilation of tree-generator module with FIR

compiler/testData/diagnostics/tests/callableReference/compatibilityResolveWithVarargAndOperatorCall.fir.kt

@@ -0,0 +1,18 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+fun interface IFoo {
+    fun foo(i: Int)
+}
+
+fun interface IFoo2 : IFoo
+
+object A
+
+operator fun A.get(i: IFoo) = 1
+operator fun A.set(i: IFoo, newValue: Int) {}
+
+fun withVararg(vararg xs: Int) = 42
+
+fun test1() {
+    <!UNRESOLVED_REFERENCE!>A[::withVararg] += 1<!>
+}

compiler/testData/diagnostics/tests/callableReference/compatibilityResolveWithVarargAndOperatorCall.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 fun interface IFoo {