FIR: Support coercion-to-unit semantics for callable references

compiler/testData/diagnostics/tests/callableReference/function/ambiguityTopLevelVsTopLevel.fir.kt

@@ -5,5 +5,5 @@ fun foo(x: Any, y: Int) = y
 fun main() {
     <!UNRESOLVED_REFERENCE!>::foo<!>
     
-    val fooRef: (Int, Any) -> Unit = <!UNRESOLVED_REFERENCE!>::foo<!>
+    val fooRef: (Int, Any) -> Unit = ::foo
 }

compiler/testData/diagnostics/tests/suspendConversion/suspendConversionWithFunInterfaces.fir.kt

@@ -17,5 +17,5 @@ fun test() {
     <!INAPPLICABLE_CANDIDATE!>foo1<!>(::bar1)
     <!INAPPLICABLE_CANDIDATE!>foo1<!>(::bar2)
 
-    <!INAPPLICABLE_CANDIDATE!>foo1<!>(<!UNRESOLVED_REFERENCE!>::bar3<!>) // Should be ambiguity
+    foo1(::bar3) // Should be ambiguity
 }