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) {}

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
-}
+}

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