[FIR2IR] Insert implicit coercion to unit in adapted function references

^KT-60260 Fixed

compiler/testData/ir/irText/expressions/funInterface/samConversionInVarargs.fir.kt.txt

@@ -33,7 +33,7 @@ fun withVarargOfInt(vararg xs: Int): String {
 fun testAdaptedCR() {
   useVararg(foos = [{ // BLOCK
     local fun withVarargOfInt(p0: Int) {
-      withVarargOfInt(xs = [p0])
+      withVarargOfInt(xs = [p0]) /*~> Unit */
     }
 
     ::withVarargOfInt /*-> IFoo */