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/callableReferenceAssignmentToVar.kt

@@ -0,0 +1,20 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+
+// FILE: lib.kt
+package lib
+fun foo(x: Double): Double = x
+
+// FILE: main.kt
+
+import lib.*
+
+fun foo(): Map<Int, String> = TODO()
+
+var z: (x: Double) -> Double = { it }
+
+fun bar() {
+    z = ::foo
+}
+
+fun baz(x: (Double) -> Double) {}