FIR: don't change explicitly given anonymous function type

compiler/testData/ir/irText/firProblems/functionLiteralGenericSignature.kt

@@ -0,0 +1,11 @@
+import java.util.Date
+
+val unitFun = { }
+val intFun = { 42 }
+val stringParamFun = { x: String -> }
+val listFun = { l: List<String> -> l }
+val mutableListFun = fun (l: MutableList<Double>): MutableList<Int> = null!!
+val funWithIn = fun (x: Comparable<String>) {}
+
+val extensionFun = fun Any.() {}
+val extensionWithArgFun = fun Long.(x: Any): Date = Date()