[SourceKit] Fix assertion failure in sortTopN

With the unqualified fallback we start to hit this assertion for
`return nil` in failable intializers. We ought to be able to just
skip over literal buckets though.

Author: hamishknight

test/SourceKit/CodeComplete/pr83662.swift

@@ -0,0 +1,7 @@
+// Make sure we don't crash
+struct S {
+  init?() {
+    return nil
+    // RUN: %sourcekitd-test -req=complete.open -pos=%(line-1):12 %s -- %s
+  }
+}

tools/SourceKit/lib/SwiftLang/CodeCompletionOrganizer.cpp

@@ -847,10 +847,7 @@ static void sortTopN(const Options &options, Group *group,
     unsigned endNewIndex = 0;
     for (unsigned i = 1; i < contents.size(); ++i) {
       auto bucket = getResultBucket(*contents[i], hasRequiredTypes);
-      if (bucket < best) {
-        // This algorithm assumes we don't have both literal and
-        // literal-type-match at the start of the list.
-        assert(bucket != ResultBucket::Literal);
+      if (bucket < best && bucket != ResultBucket::Literal) {
         if (isTopNonLiteralResult(*contents[i], best)) {
           beginNewIndex = i;
           endNewIndex = beginNewIndex + 1;