LibCpp: Handle circular imports gracefully

This patch prevents CppComprehensionEngine from endlessly looping when
there's a circular #include in the code.

We now keep track of the set of currently processed files, and will not
re-process a file if it already exists in this set.
When we're done with processing a file it is removed from this set.

The pragma once directive is not yet implemented, but regardless a
mechanism that prevents #include loops even in the absence of
pragma once is desired.

Userland/DevTools/HackStudio/LanguageServers/Cpp/CppComprehensionEngine.h

@@ -140,6 +140,11 @@ private:
     void for_each_included_document_recursive(const DocumentData&, Func) const;
 
     HashMap<String, OwnPtr<DocumentData>> m_documents;
+
+    // A document's path will be in this set if we're currently processing it.
+    // A document is added to this set when we start processing it (e.g because it was #included) and removed when we're done.
+    // We use this to prevent circular #includes from looping indefinitely.
+    HashTable<String> m_unfinished_documents;
 };
 
 template<typename Func>