FIR: pass the explicit expected type to block type

This helps avoid adding redundant return Unit into block.

compiler/testData/ir/irText/expressions/coercionToUnit.fir.kt.txt

@@ -1,6 +1,6 @@
 val test1: Function0<Unit>
   field = local fun <anonymous>(): Int {
-    return 42
+    42 /*~> Unit */
   }
 
   get