[FIR2IR] Properly set isAssignable flag for parameters of tailrec functions

^KT-63973 Fixed

compiler/testData/codegen/box/vararg/varargInTailrec.kt

@@ -0,0 +1,19 @@
+// ISSUE: KT-63973
+// WITH_STDLIB
+
+tailrec fun <T> foo(vararg items: String, fn: (String) -> T): T = when (items.size) {
+    0 -> fn("")
+    else -> foo(*items.drop(1).toTypedArray()) {
+        fn(items.first())
+    }
+}
+
+fun box(): String {
+    val result = buildString {
+        foo("abcde") { append(it) }
+    }
+    if (result != "abcde") {
+        return "Fail: $result"
+    }
+    return "OK"
+}