LanguageServers/Cpp: Add FileDB and pass project_root in Greet()

FileDB wraps the access to the contents of project files.

When asked to fetch a file, FileDB will either return its in-memory
model of the file if it has been "opened" by the language-server
protocol, or otherwise fetch it from the filesystem.

Previously, the cpp language server did not pledge "rpath" and got
access to the contents of files whenever they were opened by the
language client.
However, features like inspection of header files require the language
server to get the content of files that were not opened by the client.

The language server now pledges rpath but makes sure to only unveil
the project's directory and /usr/include.

Userland/DevTools/HackStudio/LanguageClient.cpp

@@ -62,6 +62,7 @@ void LanguageClient::remove_text(const String& path, size_t from_line, size_t fr
 
 void LanguageClient::request_autocomplete(const String& path, size_t cursor_line, size_t cursor_column)
 {
+    set_active_client();
     m_connection.post_message(Messages::LanguageServer::AutoCompleteSuggestions(path, cursor_line, cursor_column));
 }
 
@@ -78,4 +79,9 @@ void LanguageClient::set_autocomplete_mode(const String& mode)
     m_connection.post_message(Messages::LanguageServer::SetAutoCompleteMode(mode));
 }
 
+void LanguageClient::set_active_client()
+{
+    m_connection.attach(*this);
+}
+
 }