[clang-tools-extra] [clang-doc] Add a top-level index.html page (PR #171899)

[Paul Kirth via cfe-commits]

================
@@ -647,6 +648,54 @@ static SmallString<16> determineFileName(Info *I, SmallString<128> &Path) {
   return FileName;
 }
 
+// Creates a JSON file above the global namespace directory.
+// An index can be used to create the top-level HTML index page or the Markdown
+// index file.
+static Error serializeIndex(const ClangDocContext &CDCtx, StringRef RootDir) {
+  if (CDCtx.Idx.Children.empty())
+    return Error::success();
+
+  json::Value ObjVal = Object();
+  Object &Obj = *ObjVal.getAsObject();
+  if (!CDCtx.ProjectName.empty())
+    Obj["ProjectName"] = CDCtx.ProjectName;
----------------
ilovepi wrote:

Seeing this pattern again is making me wonder if we should have helper:

```c++
void insertNonEmpty(StringRef Key, StringRef Val, JSONObject Obj){
  if(!Val.empty())
    Obj[Key] = Value;
}
```

Do you think that would be useful? or make it easier to reason about?

https://github.com/llvm/llvm-project/pull/171899