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/resolve/withVararg.fir.kt

@@ -6,5 +6,5 @@ fun foo(i: Int) {}
 
 val fn1: (Int) -> Unit = ::foo
 val fn2: (IntArray) -> Unit = ::foo
-val fn3: (Int, Int) -> Unit = ::foo
-val fn4: (Array<String>) -> Unit = ::foo
+val fn3: (Int, Int) -> Unit = ::<!UNRESOLVED_REFERENCE!>foo<!>
+val fn4: (Array<String>) -> Unit = ::foo