FIR: Refine callable reference resolution in assignment position

synthetic_select<() -> T>(::foo) form allows to select better candidate
than synthetic_select(::foo) with expect type `() -> T`

compiler/testData/diagnostics/tests/callableReference/generic/expectedFunctionType.fir.kt

@@ -22,6 +22,6 @@ class A3<T> {
     fun test2(): (T) -> Unit = A3<T>()::a3
     fun test3(): (Int) -> String = A3<Int>()::a3
 
-    fun <R> test4(): (R) -> Unit = <!RETURN_TYPE_MISMATCH!>this::a3<!>
+    fun <R> test4(): (R) -> Unit = this::<!UNRESOLVED_REFERENCE!>a3<!>
     fun <R> test5(): (T) -> R = this::a3
-}
+}